Support old tradfri config format

[balloob]

Description:

Old Tradfri format had a fixed identitiy of 'homeassistant'. This adds support for it.

Related issue (if applicable): Reported by @cgtobi via Discord

Error doing job: Task exception was never retrieved

Traceback (most recent call last):
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/data_entry_flow.py", line 64, in async_init
    return await self._async_handle_step(flow, flow.init_step, data)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/data_entry_flow.py", line 98, in _async_handle_step
    result = await getattr(flow, method)(user_input)  # type: Dict
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/tradfri/config_flow.py", line 98, in async_step_import
    self.hass, user_input['host'], user_input['identity'],
KeyError: 'identity'

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code does not interact with devices:

• Tests have been added to verify that the new code works.