Initial support for Fedora 21

Initial support for Fedora 21.  Add f21 to the distribution lists for
rpms, ensure "mariadb" is used over "mysqld" and enable it in the
distro check, as we have several reports of it working.

Tested with [1]

[1] http://download.fedoraproject.org/pub/fedora/linux/releases/test/21-Beta/Cloud/Images/x86_64/Fedora-Cloud-Base-20141029-21_Beta.x86_64.qcow2

Change-Id: I97cbede806e5c00363c7174fa1e9f286de96aab6
diff --git a/stack.sh b/stack.sh
index 38ecceb..8e05ae6 100755
--- a/stack.sh
+++ b/stack.sh
@@ -143,7 +143,7 @@
 
 # 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|7.0|wheezy|sid|testing|jessie|f19|f20|rhel6|rhel7) ]]; then
+if [[ ! ${DISTRO} =~ (precise|trusty|7.0|wheezy|sid|testing|jessie|f19|f20|f21|rhel6|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"