Don't defer stopping OpenStack slave to a thread

The thread didn't do anything but poll whether the instance was deleted
or not. That's not very useful, so drop the polling and simply delete
the instance.
Don't reference self.instance in _start_instance

Don't reference self.instance, since it could be set to None by
stop_instance. This does not happen yet, but could in the future.
Merge branch 'openstack-cleanup' of git://github.com/seankelly/buildbot