Add RHEL 7 image element

This is being added as a separate element because RHEL 7 is too
different from either RHEL 6 or Fedora to share all of the
configuration they do.  It uses the redhat-common element for
pieces that are duplicated.

Some current limitations of the element:
-RHEL 7 is currently still in beta, so final image locations are
 not available.  This currently defaults to the RHEL 6.5 image,
 but that will need to be overridden using the DIB_* env vars.
-There are additional tripleo-image-elements changes that will be
 needed to be able to use this for building tripleo images.
-Open vSwitch packages are not currently available in the official
 RHEL 7 repos.
-RHEL does not support qemu without kvm acceleration, so at this
 time it cannot be used as a compute node for virtualized testing.

Change-Id: I705fd475303576525a27ce6801c673b5721792c9
Merge "Add RHEL 7 image element"