Add aggregate_host_add and _delete to conductor.
Add aggregate_host_add and _delete to conductor.

Offload the db writes for adding and removing a host to or from
a host aggregate to the conductor service.

Part of blueprint no-db-compute.

Change-Id: I7c3da2d5c9fc6d7e72fb9371b7ea6a2b10c5218d

n0ano/gantt,n0ano/gantt