Adding the ability to delete a build slave when it is released, for
cloud-based VM pools

Change-Id: Ifea493b6a93c05ad80c691dceb5886eb9cb5cad7
Reviewed-on: https://q.tasktop.com/alm/s/c2c-oss/gerrit/131
Tested-by: Hudson
Reviewed-by: Myles Feichtinger <myles.feichtinger@tasktop.com>
Reviewed-by: Michael Nelson <michael.nelson@tasktop.com>