Don't generate notifications when reaping running_deleted instances

The periodic task which looks for instances which have been deleted
but are still running at the hypervisor supports a "reap" action to
delete them. It does this by calling the _shutdown_instance() method
which as part of its processing will send notification messages.

However, the notification code depends on the instance object
including system metadata information, and the db queries that build
instance objects do not extend the "read deleted" filter down to
these queries. This results in an exception in the shutdown code
which in turn prevents the VM from being deleted.

A seperate bug has been submitted to make deleted instance objects
consistent with undeleted ones, which would fix the KeyError.

However the reap code should also not send notification messages.
The instance was deleted some time ago, and so messages will have
been generated then.  Sending new messages for the instance at this
point will confuse any billing and monitoring systems.

In the event the original shudown/delete notifictaions were lost
the absense of "instance.exists" messages already addresses this
case.

Fixes bug: 1224981

Change-Id: Id463860e3046e13ce86b472d46e1ff4a43abcee4
Merge "Don't generate notifications when reaping running_deleted instances"