Remove dependency to juju-cli for controller_name

[cderici]

Description

controllers.yaml is read for the controller_name after a connection is established, and this creates a dependency on the juju-cli to be installed in the system, which is not required for pylibjuju.

Fixes #996

QA Steps

Testing this requires a bit of stateful actions. I see two easy ways to test:

1. Use the integration test I added.

Add import pdb;pdb.set_trace() right before the new_cont.connect( line, and run the integration test with --pdb as follows:

tox -e integration -- -- tests/integration/test_connection.py::test_connection_happy_path --pdb

Before continuing, go find the controllers.yaml and rename it to something else (sort of easier than removing juju-cli etc):

mv controllers.yaml controllers.yaml.tmp

Now continue in the pdb (c) to see if the new connection will be established without a problem.

2. Do what the integration test is doing manually in the repl:

 $ python -m asyncio
>>> from juju import controller
>>> c=controller.Controller();await c.connect()

Stop at this point to rename the controllers.yaml file.

>>> new_c=controller.Controller()
>>> await new_c.connect(endpoint=c.endpoint, username=c.username, password=c.password, cacert=c.cacert)

This should succeed.

Don't forget to rename your controllers.yaml back:

mv controllers.yaml.tmp controllers.yaml

Notes & Discussion

JUJU-5317

[cderici]

/merge