Deprecate conductor's compute_stop() interface

We no longer need to run stop() actions through conductor to avoid
the instance action database hit that compute_api.stop() would
entail. This deprecates that interface and makes the manager use
the compute_api method directly.

Related to blueprint compute-api-objects

Change-Id: I11ab8fadb3e417b5256504699603e8111ed69f7f
Merge "Deprecate conductor's compute_stop() interface"