[2.2] Django interface update signal handlers are too opaque

[Mike Pontillo]

MAAS often takes action based on event-driven changes to Django models, such as pre-save, post-save, post-delete, or based on a particular field changing.

When bugs occur due to the actions these signal handlers take, they are often difficult to debug, since the work of the signal handler may be unrelated to the user's intended action.

For support purposes, logging should be added when a signal handler makes changes to the database.