Ensure the sentinel service is running

Note the commented out lines in the sentinel.pp for the config_dir
and log_file resources. If these are there we get duplicate
resources. A few options to deal with this:

* Not worry about it: the dpkg and rpm packages make sure they are
  there anyway, we're really just insuring existence of default
  things. This only becomes a problem if someone wants to write
  log files and config files in unusual places, but if that's the
  case they are probably managing that anyway.

* Extract the conflicting resources to their own classes which both
  redis and redis-sentinel use. This gets complicated in the face of
  the way redis::params is used as a super class and other
  inclusions are done.

* Require stdlib and use ensure_resource (untested, but based on
  reading the docs it seems like it ought to work).

I'm inclined to the former.

Still missing and to do:

* confirm system restart on config file changes
* flesh out the tests
* implement monitoring of multiple masters (add loop to the conf
  file template and change data structure)