cephadm: support multiple mounts when running shell

cephadm: support multiple mounts when running shell

This commit adds the multiple mounts support when running the
interactive shell.

ie:
```
--mount /foo /bar:/bar:z
```

Keeping default destination `/mnt` when no destination is passed for
backward compatibility. In the above example `/foo` will be mounted in
`/mnt/foo` and `/bar` in `/bar`

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>