Unsetting a value with "juju unset" is not reflected in the config

[Данило Шеган]

Charm helpers Config() class initializes "previous" values from a file on disk to allow for detecting changes between config values.

However, when one does a "juju unset" to remove a config value, this is never detected.

I'm attaching a branch that does what I expect, however, it breaks tests since the code has been designed to disallow unsetting of config values (once it's set, there's no way to drop it from the cache).

Note that I have tested and can confirm that juju does trigger the config-changed hook.

I've seen this while working on landscape-charm and trying to unset an explicit value of worker-counts in testing, and then attempting to unset it to restore automatic recalculation.