Password resolution in connector

[cderici]

Description

For both model and controller connections, accounts.yaml is required to have both username and password for connection to be established. This is not always true, for example, juju change-user-password actually removes the password from the
accounts.yaml file, so we pass an empty password to the connection.

This fixes #1001 by checking the credentials before trying to establish connection, and asks for whichever cred is required:

QA Steps

This will require a 2.9 controller, so bootstrap one.

 $ juju_29 bootstrap localhost issue1001

Then check the accounts.yaml file to see a line like the following for the admin, make sure the password is in there:

issue1001:
    user: admin
    password: asdasdsadasdadasdasdasdasd
    last-known-access: superuser

Now go ahead and try to connect via python-libjuju repl:

$ python -m asyncio
>>> from juju import controller
>>> c=controller.Controller()
>>> await c.connect()
>>>
exiting asyncio REPL...
# Connection works without any issues.

Change the admin user password with juju change-user-password:

$ juju change-user-password
new password: <your-password>
type new password again: <your-password>
Your password has been changed.
 $ juju users
Controller: lxd292

Name    Display name  Access     Date created   Last connection
admin*  admin         superuser  2 minutes ago  just now

Go take another look at the accounts.yaml, the password field should've disappeared.

issue1001:
    user: admin
    last-known-access: superuser

Without this change, you'd see the ssl error from #1001, because pylibjuju was trying to establish a connection with passing None as password. But with this change, first you need to see a nice error message when it fails:

$ python -m asyncio
>>> from juju import controller
>>> c=controller.Model()
>>> await c.connect()
>>>
...
...
ValueError: Some authentication parameters are required : password

Now provide your password in connect and it should connect just fine:

>>> await c.connect(password=<your-password-as-string>) # string

Notes & Discussion

JUJU-5267

[jack-w-shaw]

LGTM, QA passes

[jack-w-shaw]

Does None need to be provided in uuid, or can it be left out?