[TEST] add mapping to MoreLikeThisActionTests to make sure doc mapper exists on all shards / nodes
Add _cat/plugins endpoint

If we want to have a full picture of versions running in a cluster, we need to add a `_cat/plugins` endpoint.

Response could look like:

```sh
% curl es2:9200/_cat/plugins?v
node component                        version   type url                                   desc
es1  mapper-attachments               1.7.0       j                                        Adds the attachment type allowing to parse difference attachment formats
es1  lang-javascript                  1.4.0       j                                        JavaScript plugin allowing to add javascript scripting support
es1  analysis-smartcn                 1.9.0       j                                        Smart Chinese analysis support
es1  marvel                           1.1.0      j/s http://localhost:9200/_plugins/marvel Elasticsearch Management & Monitoring
es1  kopf                             0.5.3       s  http://localhost:9200/_plugins/kopf   kopf - simple web administration tool for ElasticSearch
es2  mapper-attachments               2.0.0.RC1   j                                        Adds the attachment type allowing to parse difference attachment formats
es2  lang-javascript                  2.0.0.RC1   j                                        JavaScript plugin allowing to add javascript scripting support
es2  analysis-smartcn                 2.0.0.RC1   j                                        Smart Chinese analysis support
```

Closes #4824.