Add group/staging support

Allow for the persona to define activation group
ordering and use that ordering to ensure that horizon
is always built in a later group (after all the other
components).

This makes horizon not suck building....

Although the following will still need to be pre
installed on rhel7 (probably rhel6 to?):

- libvirt-devel
- libffi-devel
- python-six (not the 1.3 oldness currently in rhel7)

Change-Id: Id511b84b577de0fe469b683f4a32afe8e9e4e852
Merge "Add group/staging support"