Add Excluded Users support to Git SCM

Add ability to provide a list of users to ignore revisions from
when SCM polling is enabled with Git.

Change-Id: Idb11b0afc2d8157f347e18488439d78e02f51282
Reviewed-on: https://review.openstack.org/32295
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins