List of existing plugins with Node Info API
We want to display information about loaded plugins in Node Info API using plugin option:

```sh
curl http://localhost:9200/_nodes?plugin=true
```

For example, on a 4 nodes cluster, it could output the following:

```javascript
{
  "ok" : true,
  "cluster_name" : "test-cluster-MacBook-Air-de-David.local",
  "nodes" : {
    "lodYfbFTRnmwE6rjWGGyQQ" : {
      "name" : "node1",
      "transport_address" : "inet[/172.18.58.139:9300]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/172.18.58.139:9200]",
      "plugins" : [ ]
    },
    "hJLXmY_NTrCytiIMbX4_1g" : {
      "name" : "node4",
      "transport_address" : "inet[/172.18.58.139:9303]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/172.18.58.139:9203]",
      "plugins" : [ {
        "name" : "test-plugin",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "test-no-version-plugin",
        "description" : "test-no-version-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "dummy",
        "description" : "No description found for dummy.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    },
    "bnoySsBfTrSzbDRZ0BFHvg" : {
      "name" : "node2",
      "transport_address" : "inet[/172.18.58.139:9301]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/172.18.58.139:9201]",
      "plugins" : [ {
        "name" : "dummy",
        "description" : "This is a description for a dummy test site plugin.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    },
    "0Vwil01LSfK9YgRrMce3Ug" : {
      "name" : "node3",
      "transport_address" : "inet[/172.18.58.139:9302]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/172.18.58.139:9202]",
      "plugins" : [ {
        "name" : "test-plugin",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      } ]
    }
  }
}
```
Closes #2668.