redhat: support epel yum configs in mockbuild

redhat: support epel yum configs in mockbuild

epel's mock is useful in terms of config, but we need to allow
use of it. do so here. add centos6 at the same time.

Reviewed-on: http://gerrit.openafs.org/5056
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 45f0cbf83aed2bd534e9b43822c033ef6b4a9b1b)

Change-Id: Ic46bcbcc8045773d8ce248fe2dea93b27d24ecf2
Reviewed-on: http://gerrit.openafs.org/5067
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>