Break out signalling heat-config-notify command

Currently signalling heat is done in a chunk of python at the end
of 55-heat-config. This change moves the signalling to its own
command heat-config-notify, which can be called from 55-heat-config
or anything else which wants to signal heat.

This change also adds proper test coverage to the signalling logic.

Change-Id: I6ba7468b46ca8cfad1e58207cd3f814a178ff6f1
Merge "Break out signalling heat-config-notify command"