Sentinel support

This branch adds a redis::sentinel class that, like the redis class, inherits from redis::params. Params have been updated to include the most relevant sentinel.conf setings. A spec file has been added to test the additions.

At the moment these changes allow the configuration of just one master redis server per sentinel configuration. This is to ensure that the manifests which would use the class are not complex. If multiple masters is a requirement I can update the branch accordingly.