[API][ADMIN_API][MGM]: Auto-failover code

* Refactor pool manager into a gearman worker
* Add delete and floating IP functions to pool manager
* Fix flake8 for API server
* Add new table so that admin APIs can track current builders
* Add support to Admin API to build nodes using gearman pool manager
* Add auto-delete (Nova and DB) of used devices
* Mark deleted load balancers as DELETED instead of OFFLINE/ERROR
* Move expunge handler thread from API to ADMIN_API
* Don't ERROR check all devices, just used ones
* Add vip pool scheduler
* Add vip support to node build/list/delete/floatingIP
* Use vip ID instead of device ID for API output
* Move DB and gearman from API into common section for Admin API
* Make stats thread rebuild a bad device

Change-Id: I11ee8d21610ccfdf551a0db6c4734d7fc44cced5