Update fedora docs Re: f21 release

fedora:latest now points to fedora:21 and is the official Fedora 21
docker image. fedora:20(and heisenbug) and fedora:rawhide are still
semi-official.

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>
Merge pull request #119 from fedora-cloud/fedora-21-release

Update fedora docs Re: f21 release