ASGARD-1285 - Withering

Asgard server endpoint to start Asgard ASG self-destruct "withering" process after running task count reaches zero.

While some of Asgard's long running tasks are still performed with in-memory threads instead of SWF, we need part of our deployment process to be a safe final clean up process.
* Administrator sends Asgard a REST call to start withering.
* An in-memory process starts polling for the running task count to reach zero.
* When the tasks are all done, the withering process checks that it is the only instance in its ASG. If so, the withering process performs a force delete on the disabled ASG that contains the withering Asgard instance.