Ensure GERRIT_PORT is an integer

The Gerrit port configuration option must be an integer
otherwise paramiko will complain.

Change-Id: I97d79940ceba97b1f4b3d20c4ba93b6fc4394e17
Reviewed-on: https://review.openstack.org/34121
Reviewed-by: Eric Erfanian <eric.erfanian@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins