Organize internal PostgreSQL database which will be used as DB for
Roby-on-Rails GitLab and listens only on unix socket (for security and
performance reasons - see earlier intro patch).

To do it we use slapos.cookbook:postgres recipe, with disabling
"listen-to-network" via passing empty sets to ipv4 and ipv6 recipe
arguments.

The promise to check whether DB is alive is just `psql -c '\q'` which
will error if failing to connect to DB, but exit silently if connected ok.

Explicit log rotation is not needed - as postgresql logs to
stdout/stderr - not to a file - logs are handled by slapos - put into
.slappartX_postgresql.log and automatically rotated there.

XXX omnibus-gitlab tunes postgresql with shared_buffers and other
parameters, most likely for performance reasons - see e.g.

    https://gitlab.com/gitlab-org/omnibus-gitlab/blob/8-2-stable/files/gitlab-cookbooks/gitlab/templates/default/postgresql.conf.erb#L113

I decided not to fine-tune postgresql for now, and get on-field feedback
first, and then, if needed, we can tune.

/cc @kazuhiko, @jerome