Various Sentinel fixes

[prachetasp]

This PR includes:

• The ability to make the Redis installation optional in the Sentinel setup (useful when you manage Redis installation in separate manifests/modules)
• The ability to specify the bind parameter in the Sentinel config
• Executes the Sentinel config file copy as root instead of redis. This caused subsequent Puppet runs to break since the redis user didn't have write privs on /etc/redis (this is advisable for security). Since we already specify the file should be owner => redis, group => redis specifying it again is unnecessary