Support was previously added for the other multibranch pipeline scms,
but not for gerrit. This change makes adds support for available git
extensions to the gerrit multibranch pipeline scm.
Previous review: https://review.opendev.org/c/jjb/jenkins-job-builder/+/601323
Change-Id: Ie7553ff310e844bbe912b694b09f97450c609491
Signed-off-by: Pat Long <pllong@arista.com>