(maint) Add redhat-release-everything to RHEL7 mock setup list

In RHEL7 beta, redhat-release is now redhat-release-everything and without
redhat-release-everything installed, facter doesn't quite work right.
Also, today we learned how to mix packages & group names in a single install
command. So, here's some instituional knowledge to save!
Merge pull request #61 from mckern/use_epel

(maint) Add redhat-release-everything to RHEL7 mock setup list