upstart: use exec here so upstart can monitor the process and not just a shell

Docker-DCO-1.1-Signed-off-by: Michael Gorsuch <michael.gorsuch@gmail.com> (github: gorsuch)

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