add redis resource agent script

I tried some of the existing redis resource agent scripts on here and ran into a plethora of issues so I ended up creating my own. This was written completely from scratch, not done by rewriting code from another resource agent, and as such the code is a lot simpler than the other redis scripts (less than half the size).

* It does not use `start-stop-daemon`, and thus is not dependent upon debian/ubuntu
* It properly sets the default master priority based on the `slave_priority` setting from the redis config.
* It preserves the currently running master when taking the resource from unmanaged to managed. This is important as I needed to convert a single redis server into a pacemaker master/slave cluster without shutting redis down.
* It properly starts each client up in slave mode, and wait for pacemaker to promote it to master.