Support running of secondary tracking database for GitLab Geo

Since a GitLab Geo secondary only has read-only access to the primary DB, the secondary needs some way to update its internal database to track which git repositories and which attachments it has downloaded. @dbalexandre has started working on supporting a proof of concept system in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1197.

We'll want to have omnibus support for this second database in 9.0. I think may want some easy flag to enable it, such as:

gitlab['geo_secondary'] = true

This would indicate to omnibus to start up another PostgreSQL database in another data directory. I assume we can just use 9.6 for the default here.

/cc: @brodock