Add RHEL7 support for PE

This currently requires more hackey bandaid logic (à la what we did
for the FOSS mocks for EL7). We'll address this once EL7 is released.
Update URL for RHEL7 PE mocks

Changed it to reference internal RHEL7 mirror.
Merge pull request #57 from mckern/pe_on_rhel7

Add RHEL7 support for PE