Update scheduler after instance delete

Previously deleting an instance would update the scheduler resources
fairly quickly. There is now a delay when deleting an instance until the
scheduler makes the resources available again. This appears to be due to
the fact that the delete code path used to call resource tracker to
update the compute_node record but this no longer happens.

Closes-Bug: 1336080
Change-Id: I837b70ce06843ff83481d4985c9c08b74d20cab0
Merge "Update scheduler after instance delete"