build_request not deleted when using cellsV1 and local nova_api DB

[Belmiro Moreira]

Description
===========
build_request not deleted when using cellsV1 and local nova_api

Placement needs to be enabled in Newton.
CellsV1 installations can deploy a placement service per child cell in order to have a more efficient schedule during the transition to cellV2. This requires a nova_api DB per cell.

With this configuration the "build_request" that was created in the top nova_api DB is not deleted after the VM creation because is triggered in "conductor/manager.py" that runs in the child cell and is pointing to the local nova_api DB.

This leaves new VMs in BUILD state.

Expected result
===============
build_request is removed from top nova_api DB.

Actual result
=============
nova-cells tries to remove build_request from local cell nova_api DB.

Environment
===========
Nova newton

[Belmiro Moreira (moreira-belmiro-email-lists)]

WIP patch from Dan Smith:
https://review.openstack.org/#/c/523187/