Changed flag is sometimes cleared, sometimes not

[johnsca]

The changed flag (which is also currently undocumented) can be cleared by the layer if fetch is called and either fails or there is not a new image available. This can happen if an upgrade-charm hook is fired, leading to the flag sometimes automatically clearing itself and sometimes not, which is inconsistent and prone to bugs.

Since the flag isn't even documented yet, I'm inclined to changing it so that it will always have to be explicitly cleared by the charm code and documenting that behavior.