Add group 'openstackwatch' for the openstackwatch user.

Change-Id: Ie5452b76ba0bb404899a75f666df5ad064b26dbb
Reviewed-on: https://review.openstack.org/24598
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins