Adds EPEL repo, cleans up rhel7 repos

This patch adds a new EPEL element for use with EL-based distros
in accordance with the official docs[1].  This patch also
modifies the rhel7 element to not use the hard-coded repo file.

[1] https://fedoraproject.org/wiki/EPEL

Change-Id: Ib2fa24b2d519837271a0e213a2dc5a012cc31f3c
Merge "Adds EPEL repo, cleans up rhel7 repos"