Initial support for another OS class of some sort

This will happily build the recently released RHEL 7 Beta using the "url" install method.  For reasons I've yet to sort out, it fails when trying to install using the full ISO.

Interestingly, when testing both the 0.11 release and recent upstream, I am also getting failures when doing ISO installs for F18 and F19, though they differ from the error seen with the RHEL7 beta.

In any case, this adds initial support for RHEL7 that makes it possible to build images of the Beta.  That seems worthwhile.  I submit it for your consideration.