Adding merge before build to git

This change adds the ability to merge to a specific repoistory and
branch based on the values entered.  The intent was to make the new
parameters 'remote' and 'branch' only be used if 'merge' is specified

Change-Id: Iab9db6d4ca6ff608bf946b9a38cc0756d6a78b86
Reviewed-on: https://review.openstack.org/28148
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins