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/lib/ceph b/lib/ceph
index 4d6ca4a..cbdc3b8 100644
--- a/lib/ceph
+++ b/lib/ceph
@@ -110,7 +110,7 @@
 
 # check_os_support_ceph() - Check if the operating system provides a decent version of Ceph
 function check_os_support_ceph {
-    if [[ ! ${DISTRO} =~ (trusty|f20|f21|f22) ]]; then
+    if [[ ! ${DISTRO} =~ (trusty|f21|f22) ]]; then
         echo "WARNING: your distro $DISTRO does not provide (at least) the Firefly release. Please use Ubuntu Trusty or Fedora 20 (and higher)"
         if [[ "$FORCE_CEPH_INSTALL" != "yes" ]]; then
             die $LINENO "If you wish to install Ceph on this distribution anyway run with FORCE_CEPH_INSTALL=yes"