Convert unicorn parameters to slapos and configure it to listen on unix
socket only.

( Omnibus configures unicorn to listen on unix socket and
  loopback TCP, mainly because gitlab-shell could not connect to unicorn
  via unix socket until recently:

      https://gitlab.com/gitlab-org/gitlab-shell/commit/184385ac

  But as it can now, there is no point to keep on TCP port open )

To be able to do such configuration we add stub to unicorn service
section (to create needed directories where to keep the socket).

There will be follow-up patch which configures unicorn pre/post-forking
actions, which is not trivial and thus better be done on its own.

/cc @kazuhiko, @jerome