fix(user-data): support dynamic interface for seed-deis-registry

Author: Gabriel Monroy

contrib/coreos/user-data

@@ -106,7 +106,7 @@ coreos:
 
       [Service]
       Type=oneshot
-      ExecStart=/bin/sh -c 'docker pull deis/slugrunner:latest && docker tag deis/slugrunner 172.17.8.100:5000/deis/slugrunner && docker push 172.17.8.100:5000/deis/slugrunner'
+      ExecStart=/bin/sh -c "IFACE=$(netstat -nr | grep ^0.0.0.0 | awk '{print $8}') && HOST_IP=$(/bin/ifconfig $IFACE | awk '/inet /{print $2}') && docker pull deis/slugrunner:latest && docker tag deis/slugrunner $HOST_IP:5000/deis/slugrunner && docker push $HOST_IP:5000/deis/slugrunner'
 
       [Install]
       WantedBy=multi-user.target