Add missing notifications for rescue/unrescue

The rescue and unrescue operations are missing start/end notifications for
the operation. Most Nova instance operations emit
compute.instance.<operation>.start/end notifications, which external systems
can use to track changes in the state on an instance (e.g. for billing
purposes). Rescues change launched_at for the instance, which can prevent an
external system from having a consistant picture of the instances' running,
unless they take that into account.

Change-Id: I3d7d334c88953ddf693c864c3e5e0746cdd2056c
Closes-bug: 1224088
Merge "Add missing notifications for rescue/unrescue"