Maintenance mode enter pre-flight check does not account for other nodes in maintenance mode

The checks for `microceph cluster maintenance enter` do not check if other nodes are already in maintenance mode. This means you can currently successfully run:

```
microceph cluster maintenance enter node-1
microceph cluster maintenance enter node-2
... all the nodes
```

There are checks already to try to ensure enough nodes remain active to retain quorum, but they don't account for other nodes being in maintenance mode.

This can be misleading, as in practice, it's risky to have a majority of nodes in maintenance mode.