(RE-814) Add initial support for building in a RHEL 7 mock

RHEL7 support is being added for x86_64 only; we'll fix/expand
this if upstream releases a 32-bit version of RHEL7.
Merge pull request #613 from mckern/add-el7-for-mocks

(RE-814) Add initial support for building in a RHEL 7 mock