Cannot use the consume function without having the controllers information locally

[davigar15]

Hello,

I have experienced the following error when consuming an endpoint from libjuju:

Traceback (most recent call last):
  File "test.py", line 37, in <module>
    loop.run_until_complete(libjuju.consume("admin/prometheus.grafana-source", "8f87f1bf-07cc-49fd-a80a-537197392c5c"))
  File "/usr/lib/python3.6/asyncio/base_events.py", line 484, in run_until_complete
    return future.result()
  File "/app/N2VC/n2vc/libjuju.py", line 771, in consume
    await model.consume(offer_url)
  File "/usr/local/lib/python3.6/dist-packages/juju/model.py", line 2017, in consume
    source = await self._get_source_api(offer, controller_name=controller_name)
  File "/usr/local/lib/python3.6/dist-packages/juju/model.py", line 2082, in _get_source_api
    await controller.connect(controller_name=controller_name)
  File "/usr/local/lib/python3.6/dist-packages/juju/controller.py", line 114, in connect
    await self._connector.connect_controller(controller_name, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/juju/client/connector.py", line 84, in connect_controller
    controller_name = self.jujudata.current_controller()
  File "/usr/local/lib/python3.6/dist-packages/juju/client/jujudata.py", line 137, in current_controller
    return self._load_yaml('controllers.yaml', 'current-controller')
  File "/usr/local/lib/python3.6/dist-packages/juju/client/jujudata.py", line 206, in _load_yaml
    with io.open(filepath, 'rt') as f:
FileNotFoundError: [Errno 2] No such file or directory: '/root/.local/share/juju/controllers.yaml'

This is happening because I'm running libjuju in a kubernetes pod that has neither bootstrapped the juju controller nor stored the information about it in the file-system.

[davigar15]

Fixed: #437