Use instance object for delete

I was just checking to see where db.instance_destroy() was used and it appears
that this is the last case of using the db api directly for an instance destroy.
Convert it over to use the instance object method.

This patch also includes a unit test fix.  A stub had to be updated to account
for this change.  It turns out that the test wasn't actually verifying that the
stub was being called, though.

Part of blueprint icehouse-objects

Change-Id: Iaea4e8b46c90f3762b5f458acee024e14bfa974e
Merge "Use instance object for delete"