mgr/cephadm: use dict instead of defaultdict to silence mypy warning

mgr/cephadm: use dict instead of defaultdict to silence mypy warning

otherwise we have:

1: cephadm/module.py:30: note: In module imported here:
1: cephadm/serve.py: note: In member "_check_for_strays" of class "CephadmServe":
1: cephadm/serve.py:395: error: Argument "default" to "get_metadata" of "MgrModule" has incompatible type "defaultdict[str, None]"; expected "Optional[Dict[str, str]]"

Signed-off-by: Kefu Chai <kchai@redhat.com>