Group configuration not allowing members

Seems the normalization isn't handling the inner dict case correctly.

>>> from cloudinit import distros
>>> distros._normalize_groups(x)
{'groups': [{'ubuntu': ['foo', 'bar']}, 'cloud-users']}
>>> distros._normalize_groups(x['groups'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "cloudinit/distros/__init__.py", line 431, in _normalize_groups
    c_grp_cfg[i] = []
TypeError: unhashable type: 'dict'