Atomic 2 week releases

[Haïkel]

As part of the Cloud WG, this is something we wanted to do from the beginning.

Personally, I'd prefer having two sets of images:
* unstable: based on rawhide
* testing: based on current stable + newer set of packages

The main blocker is our infrastructure, which is something that Kushal
and Mike are
trying to (partly) address by fleshing up our CI for our images.

Definitive answer will depend on what we can do with our current Rel-Eng infra.

Regards,
H.