Add support for parameterized gerrit ports.

Actually, it's support for parameterized listen_address, but the
real thing you want it for is setting the port.

Change-Id: If75fedce32f35a8f72c92fc709d5c9e8b2d35235
Reviewed-on: https://review.openstack.org/33925
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins