Catch InstanceNotFound exceptions for V2 API instance_actions

Use the common.get_instance method to get the instance object
in the V2 API instance_actions extension rather than call
the compute api directly. The utility function
handes InstanceNotFound exceptions and tranlsates them to HTTPNotFound
rather than letting them propagate up causing a traceback.

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

Change-Id: I1e69fed0c7c0c611fcfd48abbd4b4e0b1c8b403f
Closes-Bug: 1271146
Merge "Catch InstanceNotFound exceptions for V2 API instance_actions"