The flag dnf_install_epel was being used to predicate installation
of the EPEL RPM repo package, but not preventing the installation
of configuration for access to EPEL. This small patch completes
the job.
Updates the overcloud host configure CI job to install EPEL mirrors,
since it was previously relying on this buggy behaviour.
Change-Id: Ib417837d0772338b16ea9f7f2540549f277d5de8