[nova] track error migrations and orphans in Resource Tracker

Hi Nova experts,

"Not tracking error migrations and orphans in RT." is probably a bug. This may trigger some problems in update_available_resources in RT at the moment. That is some orphans or error migrations are using cpus/memory/disk etc, but we don't take these usage into consideration. And instance.resources is introduced from Train used to contain specific resources, we also track assigned specific resources in RT based on tracked migrations and instances. So this bug will also affect the specific resources tracking.

I draft an doc to clarify this bug and possible solutions: https://etherpad.openstack.org/p/track-err-migr-and-orphans-in-RT Looking forward to suggestions from you. Thanks in advance.

Best Regards, Luyao