Add /etc/default/docker support to upstart, too (mirroring sysvinit)

contrib/init/upstart/docker.conf

@@ -6,5 +6,10 @@ stop on runlevel [!2345]
 respawn
 
 script
-	/usr/bin/docker -d
+	DOCKER=/usr/bin/$UPSTART_JOB
+	DOCKER_OPTS=
+	if [ -f /etc/default/$UPSTART_JOB ]; then
+		. /etc/default/$UPSTART_JOB
+	fi
+	"$DOCKER" -d $DOCKER_OPTS
 end script