Code review comment for lp://qastaging/~danilo/charm-helpers/unset-config-change

Couple of notes:

1. Service config value always "wins" (see Config.load_previous())
2. Regarding charm-internal persistent configuration, also see `charmhelpers.core.unitdata`

I agree in hindsight that conflating the ideas of service config and arbitrary charm storage was confusing at best. charmhelpers.core.unitdata came later, and seems to be the preferred choice for simple storage within a charm, leaving the Config to be treated as read-only.