draft: Add support for branch in the channel name

[theoctober19th]

Description

This PR adds support for allowing model to deploy with track/risk/branch. Currently libjuju supports deploying only track/risk.

Fixes: #1195

QA Steps

<Commands / tests / steps to run to verify that the change works:>

tox -e py3 -- tests/unit/...

tox -e integration -- tests/integration/...

All CI tests need to pass.

<Please note that most likely an additional test will be required by the reviewers for any change that's not a one liner to land.>

Notes & Discussion

<Additional notes for the reviewers if needed. Please delete section if not applicable.>

[dimaqq]

Python built-in would be more elegant:

Suggested change

	elif len(p) == 3:
	track = p[0]
	risk = p[1]
	branch = p[2]
	elif len(p) == 3:
	track, risk, branch = p

[dimaqq]

Or, alternatively something like:

match parts:
    ...
    case [track, risk, branch]:
        pass

Ofc. refactoring is only possible if the function is covered by unit tests.