Adding support for Redis Sentinel

This is implemented in the similar way as redis-server, supporting
same feature as the other and also being able to configure Sentinel
with every parameter.

Since there is not a Sentinel package, this does not install Sentinel,
but expect to have the init scripts to manage the service.

Also, a special thing about this implementation is that the sentinel
config file is not overriden in puppet executions, this is due to
Sentinel rewrites this file everytime the is a failover process.
According to the docs it comes with Redis 2.8 only, I just checked on trusty that installs 2.8 and it is there. It would not work for previous versions. They recommend getting the binary from Redis 2.8 sentinel to use on older versions.
Feel free to use my Vagrantfile from the module to test it out. I have a few OSes listed there to validate.