Allow for custom service list at join time

Allow for a list of services to be passed to `microceph cluster join` which would override the default logic which just checks what's currently deployed and deploys until we reach 3 mon/mgr/mds.

This will need decoupling the service spawning from the joining stage as we can't feed data to the join hook.
Basically we'll need to change both `microceph cluster join` and `microceph init` (if implemented already, see #21) to make the join process use two separate client calls.