Make it possible to install sentinel independently

[tomsajan]

Pull Request (PR) description

This PR introduces the possibility to install redis::sentinel standalone, without redis::server. It comes in handy when you need your sentinels to be for example on a different node than your redis server.

The change itself is a pretty straightforward, I am just adding a require_redis parameter, that makes the requirement of redis class conditional. The default value is true, which includes the redis and it is therefore backward compatible with the current setup.

The only downside of this approach I see currently is that in case someone needs a standalone redis-sentinel from a managed repository, the repository must be added manually as the redis::preinstall is no longer included (as was previously with the redis class)

This Pull Request (PR) fixes the following issues

• installation of standalone sentinel (no open issue for this one, so far)

Thank you for any input :)