CentOS Plus is not being mirrorred by openstack-infra and contains
mostly alternative kernel images which are not expected to be used in
jobs.
Let's reconsider if there is a use case for it in the future but in
the meantime let's remove it.
Change-Id: I9681b3b92c913f45791f610d845a94ccf3a16e48
a4e9e12241