Add version to plugins

Plugin developpers can now add a version number to their es-plugin.properties file:

```properties
plugin=org.elasticsearch.test.integration.nodesinfo.TestPlugin
version=0.0.7-SNAPSHOT
```

Also, for site plugins, it's recommended to add a `es-plugin.properties` file in root site directory with `description` and `version` properties:

```properties
description=This is a description for a dummy test site plugin.
version=0.0.7-BOND-SITE
```

When running Nodes Info API, you will get information on versions:

```sh
$ curl 'http://localhost:9200/_nodes?plugin=true&pretty'
```

```javascript
{
  "ok" : true,
  "cluster_name" : "test-cluster-MacBook-Air-de-David.local",
  "nodes" : {
    "RHMsToxiRcCXwHiS6mEaFw" : {
      "name" : "node2",
      "transport_address" : "inet[/192.168.0.15:9301]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9201]",
      "plugins" : [ {
        "name" : "dummy",
        "version" : "0.0.7-BOND-SITE",
        "description" : "This is a description for a dummy test site plugin.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    },
    "IKiUOo-LSCq1Km1GUhBwPg" : {
      "name" : "node3",
      "transport_address" : "inet[/192.168.0.15:9302]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9202]",
      "plugins" : [ {
        "name" : "test-plugin",
        "version" : "0.0.7-SNAPSHOT",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      } ]
    },
    "H64dcSF2R_GNWh6XRCYZJA" : {
      "name" : "node1",
      "transport_address" : "inet[/192.168.0.15:9300]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9200]",
      "plugins" : [ ]
    },
    "mGEZcYl8Tye0Rm5AACBhPA" : {
      "name" : "node4",
      "transport_address" : "inet[/192.168.0.15:9303]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9203]",
      "plugins" : [ {
        "name" : "test-plugin",
        "version" : "0.0.7-SNAPSHOT",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "test-no-version-plugin",
        "version" : "NA",
        "description" : "test-no-version-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "dummy",
        "version" : "NA",
        "description" : "No description found for dummy.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    }
  }
}
```

Relative to elastic#2668.
Closes elastic#2784.

src/main/java/org/elasticsearch/action/admin/cluster/node/info/PluginInfo.java

@@ -18,6 +18,8 @@
  */
 package org.elasticsearch.action.admin.cluster.node.info;
 
+import org.elasticsearch.Version;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Streamable;
@@ -35,12 +37,15 @@ static final class Fields {
         static final XContentBuilderString URL = new XContentBuilderString("url");
         static final XContentBuilderString JVM = new XContentBuilderString("jvm");
         static final XContentBuilderString SITE = new XContentBuilderString("site");
+        static final XContentBuilderString VERSION = new XContentBuilderString("version");
+        static final String VERSION_DEFAULT = "NA";
     }
 
     private String name;
     private String description;
     private boolean site;
     private boolean jvm;
+    private String version;
 
     public PluginInfo() {
     }
@@ -52,12 +57,18 @@ public PluginInfo() {
      * @param description Its description
      * @param site        true if it's a site plugin
      * @param jvm         true if it's a jvm plugin
+     * @param version     Version number is applicable (NA otherwise)
      */
-    public PluginInfo(String name, String description, boolean site, boolean jvm) {
+    public PluginInfo(String name, String description, boolean site, boolean jvm, String version) {
         this.name = name;
         this.description = description;
         this.site = site;
         this.jvm = jvm;
+        if (Strings.hasText(version)) {
+            this.version = version;
+        } else {
+            this.version = Fields.VERSION_DEFAULT;
+        }
     }
 
     /**
@@ -91,7 +102,7 @@ public boolean isJvm() {
     /**
      * We compute the URL for sites: "/_plugin/" + name + "/"
      *
-     * @return
+     * @return relative URL for site plugin
      */
     public String getUrl() {
         if (site) {
@@ -101,6 +112,13 @@ public String getUrl() {
         }
     }
 
+    /**
+     * @return Version number for the plugin
+     */
+    public String getVersion() {
+        return version;
+    }
+
     public static PluginInfo readPluginInfo(StreamInput in) throws IOException {
         PluginInfo info = new PluginInfo();
         info.readFrom(in);
@@ -113,6 +131,9 @@ public void readFrom(StreamInput in) throws IOException {
         this.description = in.readString();
         this.site = in.readBoolean();
         this.jvm = in.readBoolean();
+        if (in.getVersion().onOrAfter(Version.V_1_0_0_RC2)) {
+            this.version = in.readString();
+        }
     }
 
     @Override
@@ -121,12 +142,16 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeString(description);
         out.writeBoolean(site);
         out.writeBoolean(jvm);
+        if (out.getVersion().onOrAfter(Version.V_1_0_0_RC2)) {
+            out.writeString(version);
+        }
     }
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         builder.startObject();
         builder.field(Fields.NAME, name);
+        builder.field(Fields.VERSION, version);
         builder.field(Fields.DESCRIPTION, description);
         if (site) {
             builder.field(Fields.URL, getUrl());
@@ -157,4 +182,15 @@ public int hashCode() {
         return name.hashCode();
     }
 
+    @Override
+    public String toString() {
+        final StringBuffer sb = new StringBuffer("PluginInfo{");
+        sb.append("name='").append(name).append('\'');
+        sb.append(", description='").append(description).append('\'');
+        sb.append(", site=").append(site);
+        sb.append(", jvm=").append(jvm);
+        sb.append(", version='").append(version).append('\'');
+        sb.append('}');
+        return sb.toString();
+    }
 }