Update Nova's available resources at termination

This updates ClusteredComputeManager to override terminate_instance() with
a call to update_available_resource().  The goal here is to limit the lag
between when an Ironic node has been freed and when it will become available
again to the Nova.  This avoids scheduling errors against Ironic node
inventories with high utilization and/or limited node resources.

Change-Id: I767cb97d9b182f1f9528d078b07a85ffbad313ba
Closes-bug: #1334027
Merge "Update Nova's available resources at termination"