Remove the delorean-repo element from CentOS images
Remove the delorean-repo element from CentOS images

The delorean-repo element is not needed. TripleO used DIB_YUM_REPO_CONF
and RDO injects the repos into the base image prior to using
diskimage-builder.

Change-Id: I4c7e211c47dcfc274ccf7a7116da2ef63ef0268b

openstack/tripleo-common,openstack/tripleo-common