Honor the `--cluster` flag for `mon` actions

Ticket 5483 describes a situation where regardless of using `--cluster` any `mon` action will just use `ceph` as the default. 

This is basically because none of the paths are set to use the cluster value that is passed onto the functions that perform those actions.

This pull request addresses that problem, but also creates a better foundation for dealing with ceph paths. A new utility module is introduced with a `paths` module that (for now) contains `mon` helpers for `init`, `done`, `keyring` and base paths.

By abstracting paths in this way we do not need to repeat paths (and possibly miss something out) all over the place.

Tests where added and some local testing with monitor creation was performed.