fix list index error when no ':' exists in api key

[adam-stokes]

  File "/usr/lib/python3/dist-packages/maasclient/__init__.py", line 130, in nodes
    res = self.get('/nodes/', dict(op='list'))
  File "/usr/lib/python3/dist-packages/maasclient/__init__.py", line 57, in get
    auth=self._oauth(),
  File "/usr/lib/python3/dist-packages/maasclient/__init__.py", line 44, in _oauth
    resource_owner_key=self.auth.token_key,
  File "/usr/lib/python3/dist-packages/maasclient/auth.py", line 60, in token_key
    return self.api_key.split(':')[1] if self.api_key else None
IndexError: list index out of range