Add stub text for USG and system CLAs in Gerrit.

Change-Id: I0743a79c5288df4725d8f27227738019c875f91c
Reviewed-on: https://review.openstack.org/23385
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins