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/files/rpms/general b/files/rpms/general
index 7b2c00a..67b084e 100644
--- a/files/rpms/general
+++ b/files/rpms/general
@@ -25,6 +25,6 @@
 libyaml-devel
 gettext  # used for compiling message catalogs
 net-tools
-java-1.7.0-openjdk-headless  # NOPRIME rhel7,f20
+java-1.7.0-openjdk-headless  # NOPRIME rhel7
 java-1.8.0-openjdk-headless  # NOPRIME f21,f22
 pyOpenSSL # version in pip uses too much memory