Objectify instance_action for cell scheduler

This is a refactor change to make cell scheduler use InstanceAction
class method instead of compute_utils, so that all instance action can
be manipulated through object model and instance action related code in
compute_utils can be deprecated.

This is a part of: blueprint make-instance-action-use-objects
Change-Id: I920c8bf809200e6fc2c0329c11ea4be94dc71158
Merge "Objectify instance_action for cell scheduler"