add-model without --credential option updates remote credential content

[Anastasia]

Currently, when we add a hosted model via 'juju add-model' command, we implicitly update cloud credential content on the controller based on the client. This is unexpected and is a source of confusion to a user.

When 'add-model' is issued without a credential option, we should use the default credential for this user stored on the controller.

When 'add-model' is issued with a credential option:
* if there is a credential with the same name on the controller, update its content using credential from the client;
* if there is no credential with the same name on the controller, upload it from the client store.

[Anastasia (anastasia-macmood)]

Quick fix PR against develop: https://github.com/juju/juju/pull/9312

Additional fix will be followed shortly. We need to separate AddCredential from UpdateCredential at the API layer.

[Anastasia (anastasia-macmood)]

The above PR has been Fix Committed.

I'll keep this bug for part 2 as described.

[Anastasia (anastasia-macmood)]

2nd part of the fix is to call the AddCredential method on Cloud facade rather than UpdateCredential.
PR against develop: https://github.com/juju/juju/pull/9342