Fix up EPEL element
Fix up EPEL element

For whatever reason, RHEL identifies itself with DISTRO "rhel" for 6
and "rhel7" for 7, but centos just uses "centos" and DIB_RELEASE.  So
this was wrong and installing EPEL6 on centos7.

But we can simplify it completely for centos because that comes with
the epel-release package already included.

Change-Id: I2b8f5d30b850fef46b4a5ba32a917abcbf25932c

openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder