This change converts our use of CentOS7/88 to UBI8, which should
provide a better test environment which will match that of
production for future releases of TripleO. While this image change
will better match production clouds thanks in large part to TCIB,
it will also remove our use of docker.io (dockerhub) which fraught
with peril due to the ongoing API rate limits saga.
Change-Id: Ide53bef507b02b35581f596443c8a8dfa5bc7083
a89c63ba16