build_facades logic doesn't account for discharge-required-error

[johnsca]

When connecting to JAAS, sometimes one of the endpoints will not have a discharged macaroon and we'll need to try another one. This isn't handled properly by the build_facades logic, and leads to this failure:

2017-04-26 12:28:44,633 [INFO] conjure-up/ghost - websocket: connection.py:161 - Driver connected to juju wss://1.us-east-1.aws.jaas.jujucharms.com:443/model/0b51d501-1f74-4ca2-800d-0461c0c6eef9/api
2017-04-26 12:28:44,745 [INFO] conjure-up/ghost - websocket: connection.py:440 - {'discharge-required': {'caveats': [{'cid': 'time-before '
                                            '2017-04-26T17:28:44.728481186Z'},
                                    {'cid': '...',
                                     'cl': 'https://api.jujucharms.com/identity',
                                     'vid': '...'}],
                        'identifier': 'api-login',
                        'location': 'juju model '
                                    'd5549d83-423c-43c7-8a87-238a312dcf18',
                        'signature': '...'},
 'discharge-required-error': 'verification failed: macaroon not found in '
                             'storage'}
2017-04-26 12:28:44,745 [ERROR] conjure-up/ghost - juju.model: model.py:648 - Error in watcher
Traceback (most recent call last):
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/client/connection.py", line 442, in build_facades
    self.facades = VERSION_MAP[self.info['server-version']]
KeyError: 'server-version'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/model.py", line 624, in _start_watch
    self._watch_conn = await self.connection.clone()
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/client/connection.py", line 295, in clone
    self.loop,
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/client/connection.py", line 328, in connect
    await client.login(username, password, macaroons)
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/client/connection.py", line 469, in login
    self.build_facades(response.get('facades', {}))
  File "/home/johnsca/juju/conjure-up/conjure-dev/lib/python3.5/site-packages/juju/client/connection.py", line 446, in build_facades
    self.info['server-version']))
KeyError: 'server-version'
2017-04-26 12:29:54,745 [INFO] conjure-up/ghost - websockets.protocol: protocol.py:618 - Failing the WebSocket connection: 1006