Remove Fedora 20 as supported distribution

Fedora 20 will reach end of life on 23-JUN-2015[1]; remove it as
supported distribution. Add Fedora 22 where applicable.

    - stack.sh: Remove Fedora 20 from list of supported distributions.
    - files/rpms/general: Remove 'f20' from NOPRIME.
    - lib/ceph: Remove 'f20' from the check_os_support_ceph()
      function.
    - doc/source/index.rst: s/Fedora 20/Fedora 21/

[1] https://lists.fedoraproject.org/pipermail/devel-announce/2015-May/001586.html

Change-Id: I8f2e1ddc24c071754b1cceb5bed5bdafdc9d9f79
diff --git a/stack.sh b/stack.sh
index dc79fa9..0c01165 100755
--- a/stack.sh
+++ b/stack.sh
@@ -173,7 +173,7 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (precise|trusty|utopic|vivid|7.0|wheezy|sid|testing|jessie|f20|f21|f22|rhel7) ]]; then
+if [[ ! ${DISTRO} =~ (precise|trusty|utopic|vivid|7.0|wheezy|sid|testing|jessie|f21|f22|rhel7) ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"