Add a job that runs gitdm on commits.

Change-Id: Iac6a4073cc4a2569fdbc190a614d6eeebb7a2284
Reviewed-on: https://review.openstack.org/31296
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins