Prune node stats at compute node delete time

This commit addresses the situation in which a compute node is deleted, but
its compute node stats still remain in the database as **not** deleted. This
causes ill side effects in compute_node_get_all when it's retrieving host stats
as it doesn't expect there to be compute node stats for which there is no
corresponding compute node (i.e., causing some nodes' stats to be empty).

As such, when a compute node is deleted, its stats should also be implicitly
deleted.

The new test case that's been created fails without the code changes, which
illustrates the problem that compute node stats are empty when they should
not be.

Also included is a simple DB migration script that will update old stats that
were not marked soft-deleted as they should have been.

Change-Id: Ief0f7cf1a506e71898b5a45a0513d34167432d67
Closes-Bug: #1224712
Merge "Prune node stats at compute node delete time"