We tweak database.yml to point to our postgresql unix socket; set
adapter to hardcoded postgresql, encoding to unicode and omit collation
(which according to omnibus-gitlab is used for mysql only).

The only instance parameter imported from omnibus is `db_pool` - how
many connection to a DB to keep open in a RoR thread/process.

XXX we use db's superuser as a user to connect. Is it ok to do even if
    the whole DB is used only for gitlab? (I think it is ok for the
    first iteration, but we'll probably need to refine this later)

/cc @kazuhiko, @jerome