Update the supported Fedora releases
Update the supported Fedora releases

Hammer LTS 0.94.6 is currently available for Fedora 22 and 23 [1].

Fedora 21 is EOL and should be removed from the list [2].

[1] https://admin.fedoraproject.org/pkgdb/package/rpms/ceph/
[2] https://fedoraproject.org/wiki/Releases

Change-Id: I5fc7f25ed145c29cadbe03d945511aaa12c54310

ajarr/devstack-plugin-ceph,openstack/devstack-plugin-ceph