Deprecate conductor's compute_reboot() interface

We no longer need to run reboot() 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: I86c70fb33b023d55d88a64de1fa6c39c10fd7b01
Merge "Deprecate conductor's compute_reboot() interface"