Fedora 21 release image, point fedora:latest to 21

The official fedora 21 image koji build can be found here:
http://koji.fedoraproject.org/koji/taskinfo?taskID=8289768

This commit also points fedora:latest to fedora:21

Update URLs to point to fedora-cloud account repo.

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>