Make openstack mirror optional.

While we debug mirror creation.

Change-Id: Iae00bacded97d1d59ee3a48c3aa75d6b87708821
Reviewed-on: https://review.openstack.org/24712
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins