Add pack_action_start/finish helper to InstanceAction object

Add these two helper function to cut InstanceAction object's dependency
on compute/utils.py. This is a refactor change so that
pack_action_start/finish code in compute/utils.py can be removed in a
later change.

The version of InstanceAction object doesn't change, since it's just an
object internal refactoring. No object field or method has been changed.

This is a part of: blueprint make-instance-action-use-objects

Change-Id: I38bde7b057efa0ce5634941f4ffa5ef27123958c
Merge "Add pack_action_start/finish helper to InstanceAction object"