Allow handle_signal to assert which actions are valid

This retains the current behaviour for InstanceGroup and Restarter
while allowing other resources to apply different policies about
what actions are allowed when a signal is received. This is needed
by SoftwareDeployment, which can legitimately receive signals
during SUSPEND and DELETE.

Change-Id: I6f694451669b6c9b60b9fbd3ad77d0fc111bbb5a
Closes-Bug: #1286920
Merge "Allow handle_signal to assert which actions are valid"