ceph-defaults: Enable local epel repository

ceph-defaults: Enable local epel repository

During the tests, the remote epel repository is generating a lots of
errors leading to broken jobs (issue #2666)

This patch is about using a local repository instead of a random one.
To achieve that, we make a preliminary install of epel-release, remove
the metalink and enforce a baseurl to our local http mirror.

That should speed up the build process but also avoid the random errors
we face.

This patch is part of a patch series that tries to remove all possible yum failures.

Signed-off-by: Erwan Velu <erwan@redhat.com>