Updating 'source' sometimes does nothing

[Данило Шеган]

When you update 'source' on landscape-server charm with 'juju set landscape-server source=...', it will behave weirdly after the first run.

For instance, if originally deployed with source 'ppa:landscape/14.10' and then set to 'ppa:landscape/15.01', attempting to re-set again to a new value will result in ppa:landscape/15.01 remaining in the /etc/apt/sources.list.d/. If you ever attempt to set back to original package archive, it will not be added.

The problem is that we are not storing changed values for config in the config hook, so the initial value will be config.previous('source') forever.