Merge pull request #4879 from gorsuch/exec-upstart

Use "exec" in the upstart script to let upstart manage the docker process instead of a shell

contrib/init/upstart/docker.conf

@@ -37,5 +37,5 @@ script
 	if [ -f /etc/default/$UPSTART_JOB ]; then
 		. /etc/default/$UPSTART_JOB
 	fi
-	"$DOCKER" -d $DOCKER_OPTS
+	exec "$DOCKER" -d $DOCKER_OPTS
 end script