Make aggregate host operations use Aggregate object

This removes use of the conductor aggregate host methods, which
can now be done on the aggregate object itself. This is only used
in the xenapi driver, and the tests validate the operation in the
database afterwards, so no test changes are required here. This
will allow us to deprecate those aggregate methods in the conductor
api.

Related to blueprint compute-manager-objects-juno

Change-Id: I78f8d0d9ad0dd73c5c52582940dd70db73522600
Merge "Make aggregate host operations use Aggregate object"