Return the logstash build sha1 in the root api.

In our API response call definition for our root resource ("/") we have discussed (it could have been in zoom, so I cannot find why we wanted that) to include the build hash with the logstash version. This feature require to update our build process to save that information in a ruby constant that the api could return. ref #4446

``` json
{
"hostname": "skywalker",
"version" : {
"number" : "2.1.0",
"build_hash" : "72cd1f1a3eee09505e036106146dc1949dc5dc87"
}
}
```

Is this feature really necessary? For production server the current version should be enough? People shouldn't run their production server from master.