Elasticsearch integration

We added an option to use elastic search instead of database means to search everything. This option will be available for EE only. User should have a choice if he/she wants to use it, by default it should be disabled. GitLab application will expect elasticsearch to be ran if it is enabled in the gitlab.yml (see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/109)

## Elasticsearch (EE only)
elasticsearch:
  host: localhost
  port: 9200

We are going to implement all this stuff for 8.4 release. Please don't merge anything until the origin request will be merged.

@marin @dzaporozhets