Add _cat/versions endpoint

If we want to have a full picture of versions running in a cluster, we need to add a `_cat/versions` endpoint.

Response could look like:

```sh
% curl es2:9200/_cat/versions?v
node component                        version
es1  jvm                              1.6.0_65-b14
es1  elasticsearch                    0.90.10
es1  elasticsearch-mapper-attachment  1.7.0
es1  elasticsearch-lang-js            1.7.0
es1  elasticsearch-analysis-smartcn   1.9.0
es2  jvm                              1.6.0_65-b14
es2  elasticsearch                    1.0.0.RC1
es2  elasticsearch-mapper-attachment  2.0.0.RC1
es2  elasticsearch-lang-js            2.0.0.RC1
es2  elasticsearch-analysis-smartcn   2.0.0.RC1
```

Closes #4824.

**Note** that this PR requires first #4378 to be merged.