Bring up Fedora 21 nodes on rackspace and hpcloud. Although we are
close to being able to build our own images
(Ic6c21f715441a16a1d832163dc71921f25d654df), things aren't quite there
yet.
In the mean time, let's use upstream F21 images for initial testing to
ensure we don't regress while it is being worked on.
I envisage that the existing F20 devstack jobs will disappear quickly
for this once we've got this stable.
Change-Id: If764f676f43c86830b4277adaaf71098e9d4140b
cba47056ab