Make the return value of `SubiquityController.interactive` an explicit
False return if the controller is not interactive.

Previously, if none of the `if` blocks passed then `interactive` would
implicility return `None`, which would effectively return `False` when
used in `if controller.interactive()` checks. Let's make it explicit.

Additionally, add a docstring that describes the ways in which the _active
value can be changed and generally when controllers get configured.

Lastly, add some tests to the controller class to assert behavior about
interactivity and the interactive_for_variants functionality.

Co-authored-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

Fix a mismatch between the variant of the server and the variant of the
SubiquityModel. POST requests to /meta/client_variant would modify the
variant for both the server and the SubiquityModel, while POST requests to
the source controller would update only the SubiquityModel.

Fix this by making all requests to change the variant go through a
single function in the server and let that be responsible for updating
itself and the SubiquityModel.

(Conceptually the Source controller should not be responsible for
updating the SubiquityModel.)

When the source controller starts in the server, it reads the source
catalog and sets the currently selected version to the first item in the
list. Subiquity server starts with a default "server" variant, but needs
to be updated on startup if the install media doesn't match that expectation.