Catch InstanceNotFound exceptions for V2 API deferred delete

Adds handling of InstanceNotFound exceptions in the V2 API
deferred delete and translate them to HTTPNotFound rather
than letting them propagate up causing a traceback in the
nova log.

Co-Authored-By: Michael Still <mikal@stillhq.com>

Change-Id: I3d434f28313ae633bdcb9cef82a7b8b0020b5722
Closes-Bug: 1270803
Merge "Catch InstanceNotFound exceptions for V2 API deferred delete"