Make compute manager use InstanceInfoCache object for deletes

This patch makes compute manager use the InstanceInfoCache object
that is already in the instance to delete the cache while deleting
the instance.

It also fixes two tests that were still passing non-object
instance dicts to the method, which wasn't reflective of reality.

Related to blueprint compute-manager-objects

Change-Id: I829bd2d3828c01c18f3479375bea22e7f4be9d97
Merge "Make compute manager use InstanceInfoCache object for deletes"