object-ify availability_zones

This change converts availability_zones to use the Aggregate and
Service objects instead of direct database access.

Related to blueprint compute-manager-objects-juno

Change-Id: I2ec7373f063ed728dde2af3b7c8259f4391885bc
Fix error status code for agents

When passing bad body in a request, most APIs return BadRequest response.
However, agents API doesn't do it.
This patch fixes the error status code and adds a unit test related to
this change.

This patch's idea most came from
Ken'ichi Ohmichi's patch https://review.openstack.org/#/c/107266/

Change-Id: I9602183ade4d3c0105e2498aebb7e33eb72f86f0
Remove unused cell_scheduler_method

This key isn't used anywhere in the code (it's use was removed a while ago).

Change-Id: Icf215b5dd0c7b93b336b6bb030f3a9282a0b05bd
Merge "Fix error status code for agents"
Merge "object-ify availability_zones"
Merge "Remove unused cell_scheduler_method"