This new version is:
* Faster (multithreaded, thanks to a ThreadPoolExecutor)
* Safe (we check and wait for some prerequisites before attempting a
delete)
* 100% unit tested.
* Modular (one file per service)

Note that it's Python3.5 compatible. It also uses OpenStack Shade
and OpenStack client-config libraries so that OSPurge focuses on the
cleaning logic only.