ElasticsearchIllegalStateException when invoking _cat plugins

Reproducible using ES 1.1.0.

`/_cat/plugins` works fine when no plugins are installed.

But once a plugin is installed (eg. latest marvel), it throws an exception indicating that the resulting table constructed has 7 fields in the header row but not enough fields in the data rows to make it a valid table.

```
{
  "status" : 500,
  "error" : "ElasticsearchIllegalStateException[mismatch on number of cells 6 in a row compared to header 7]"
}
```

This issue has been introduced when we removed `isolation` feature in 1.1 branch. See c34d8bd.

Closes #5715

src/main/java/org/elasticsearch/rest/action/cat/RestPluginsAction.java

@@ -86,7 +86,6 @@ Table getTableWithHeader(final RestRequest request) {
         table.addCell("component", "alias:c;desc:component");
         table.addCell("version", "alias:v;desc:component version");
         table.addCell("type", "alias:t;desc:type (j for JVM, s for Site)");
-        table.addCell("isolation", "alias:i;desc:isolation");
         table.addCell("url", "alias:u;desc:url for site plugins");
         table.addCell("description", "alias:d;default:false;desc:plugin details");
         table.endHeaders();