Use unversioned RHEL element w/ RHEL8
Use unversioned RHEL element w/ RHEL8

Using 'rhel8' instead of 'rhel' causes errors.

Related to commits:
  18788b2b1748ffaa088a23caeec165391d9d7b4e
  5877ef54d3c3a2392297123a70cbf4a03f93e977

Change-Id: I54097b2bbf19cd07bceb6ce608aa7f80435ad894
Signed-off-by: Lon Hohberger <ff1811917d4a4d5f8c801f25e2c8fea5684c1092@redhat.com>

openstack/tripleo-common,openstack/tripleo-common