Generated passwords can violate ceph dashboard password policies

We're generating a random password thusly:

```
    def _gen_user_password(self, length: int = 12) -> str:
        """Generate a password"""
        alphabet = (
            string.ascii_lowercase + string.ascii_uppercase + string.digits)

        return ''.join(secrets.choice(alphabet) for i in range(length))
```

https://opendev.org/openstack/charm-ceph-dashboard/src/branch/master/src/charm.py#L590

But with that randomly generated passwords we could run afoul the ceph dashboard policies, e.g. in CI I've seen it hit this:

https://github.com/ceph/ceph/blob/main/src/pybind/mgr/dashboard/services/access_control.py#L114