[elastic] tag stats metric with the node name 🏷

When the `cluster_stats` parameter is enabled in the user configuration,
the Datadog Agent's Elasticsearch check attempts to resolve and
attach with the corresponding metrics, the hostname of the different
nodes.

On Amazon Elasticsearch v1.5.2, the node objects
returned by the `_cluster/nodes/stats?all=true` endpoint does not return
this information. Instead the following keys are available:
```
root@xxxx:~# at node_stats.json| jq '.nodes[] | select(.name ==
"${hostname}") | keys
[
  "breakers",
  "fs",
  "indices",
  "jvm",
  "name",
  "os",
  "process",
  "thread_pool",
  "timestamp"
]
```

`name` is a unique piece of identity. It generally match the hostname.
Use it an additional tag with the namespace `node_name`.

checks.d/elastic.py

@@ -505,19 +505,29 @@ def _process_pending_tasks_data(self, data, config):
 
     def _process_stats_data(self, nodes_url, data, stats_metrics, config):
         cluster_stats = config.cluster_stats
-        for node_name in data['nodes']:
-            node_data = data['nodes'][node_name]
-            # On newer version of ES it's "host" not "hostname"
-            node_hostname = node_data.get(
-                'hostname', node_data.get('host', None))
+        for node_data in data['nodes'].itervalues():
+            metric_hostname = None
+            metrics_tags = list(config.tags)
+
+            # Resolve the node's name
+            node_name = node_data.get('name')
+            if node_name:
+                metrics_tags.append(
+                    u"node_name:{}".format(node_name)
+                )
 
-            # Override the metric hostname if we're hitting an external cluster
-            metric_hostname = node_hostname if cluster_stats else None
+            # Resolve the node's hostname
+            if cluster_stats:
+                for k in ['hostname', 'host']:
+                    if k in node_data:
+                        metric_hostname = node_data[k]
+                        break
 
             for metric, desc in stats_metrics.iteritems():
                 self._process_metric(
-                    node_data, metric, *desc, tags=config.tags,
-                    hostname=metric_hostname)
+                    node_data, metric, *desc,
+                    tags=metrics_tags, hostname=metric_hostname
+                )
 
     def _process_pshard_stats_data(self, data, config, pshard_stats_metrics):
         for metric, desc in pshard_stats_metrics.iteritems():