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.
Fixed for the code review

Adding upstart init script
Config file can be replace if sentinel is restarted

It is not a problem if the config is overriden, sentinel will update
it again once it start running
Bugfix, notify instead of subscribe
Sentinel can also be used without redis-server, and needs the /etc/redis directory for Debian
To let Sentinel work autonomously, we must be sure the redis user exists
More directories that must exists to be autonomous
Avoiding service refresh everytime puppet runs

Sentinel rewrites the config file and that makes puppet overwrite it
always causing a sentinel restart.
With this fix that won't happen again, this manages the service in a
extra file that has the original contents, it it changes, then copy it
to the real config file and restarts sentinel
Merge github.com:victorgp/puppet-redis