ElasticSearch configuration via `gitlab.rb`

Summary

We would like to be able to control aspects of ElasticSearch (for starters, the ElasticSearch URL) via gitlab.rb. Support for this via the ELASTIC_URL environment variable is already in gitlab-rails, so it is a matter of setting that environment variable.

Proposal

The majority of GitLab's functionality can be configured in the Omnibus using gitlab.rb; however ElasticSearch can only be configured via the admin UI. This is generally OK, but in high availability scenarios where instances auto-configure themselves on boot, that gitlab.rb cannot configure these settings make self-healing a little more difficult. Also, scenarios where we may need to reproduce a production environment for testing via snapshots/database dumps would work, without the new environment referring to the old production ElasticSearch cluster.

Ideally, we should be able to set gitlab_rails['elastic_url'] or the like in gitlab.rb.