Cells: Fix instance deletes

A regression was introduced with object conversion that caused all
instances to be locally deleted in the API cell when cells is enabled.
This would happen before the child cell was told to delete the instance.

Additionally, if an instance record was created in a child cell but
failed to schedule to a host in such a way that the API cell has
no knowledge of the 'host', the instance would be deleted in the API
cell only.

Also, it appears that we would cast deletes to child cells twice due to
logic duplicated in cells_api.py that exists in api.py.

Closes-Bug: #1240247

Change-Id: I94cfddd9cef2d63b2cb31c0f2dae8e211a4418d9
(cherry picked from commit 0493c803ae9612f87ed028e1a39e880aead5bdcb)
Merge "Cells: Fix instance deletes" into stable/havana