Fix nova DB 215 migration script logic error

This addresses a minor (albeit important) logic error in the migration script
for https://review.openstack.org/#/c/46379/ - that script currently deletes
compute node stats for active compute nodes.  This patch reverses that logic
so it deletes the correct set of compute node stats.

Also included is a migration test case to demonstrate the behavior.

Change-Id: I77afcf443357d0767ac933d791a565289eabee9a
Closes-Bug: #1238281
(cherry picked from commit c3330931113ac2edf5961a653e5c2cfe459c13a0)