Added parameter to enable Unicorn to listen on give IP address

Closes: #196

(cherry picked from commit 5ad666d700366ec3a5971f3bd43f047b7dbc0390)
(cherry picked + squash from commit 8061750231d2eaaf992dbc22bbe2d5fdfe3d58a8)

Spec-tests fixed by sbadia, see
https://github.com/sbadia/puppet-gitlab/pull/196#issuecomment-61342469