CentOS: Fix EPEL mirroring and RDO install on CI nodes
CentOS: Fix EPEL mirroring and RDO install on CI nodes

CentOS tests have reverted to using upstream for EPEL rather than
local mirrors, introducing some unnecessary instability.  The root of
the problem is that /etc/nodepool/provider disappeared with zuulv3, so
we now always re-install the EPEL repo and overwrite the local EPEL
.repos files that were made during test setup and point to local
mirrors.

The other change is that we stopped installing the RDO repositories on
the testing nodes too.  That we were incorrectly taking this path and
reinstalling EPEL has hidden the removal of these packages from the
base image in the test, since it ends up installing them too.

Split the install into two parts -- epel and RDO.  Check for
/etc/ci/mirror_info.sh (the sourcable mirror script provided by base
test setup) and if so, just enable EPEL so we get the CI-mirror
version correctly.  Install the RDO repositories (if not already
installed) unconditionally.

Change-Id: Iccb045a6695deb10da4d68a5694e1fa45ccbb810

noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,dtroyer/devstack,group-policy/devstack,group-policy/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,nawawi/openstack