Add EPEL support.

While testing centos5/6 I realized it was going to fail as we require
EPEL to be enabled and the testing images don't have it.  Add
`epel_enable` as an option in case people are already handling epel
in another way.

This fixes the centos6 case which didn't work out of the box.

(I don't know if you're OK with this defaulting to on, but I figure I'll just submit and let you decide on that front.)