Ensure a reliable way to kill ghost containers on reboot

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

runtime/runtime.go

@@ -192,7 +192,7 @@ func (runtime *Runtime) Register(container *Container) error {
 				if err != nil {
 					utils.Debugf("cannot find existing process for %d", existingPid)
 				}
-				runtime.execDriver.Kill(cmd, 9)
+				runtime.execDriver.Terminate(cmd)
 			}
 			if err := container.Unmount(); err != nil {
 				utils.Debugf("ghost unmount error %s", err)