el6: explicitly require epel

Vdsm requires packages delivered via EPEL-6. This change makes the
dependency explicit on the product level.

Bug-Url: https://bugzilla.redhat.com/1018947
Change-Id: I0859591b3a5fcabfa3000aae6967cfef6116edda
Signed-off-by: Dan Kenigsberg <danken@redhat.com>