Openstack nova Instance failed to push static IP through config drive. openstack/content/0000 is missing

So I need to bootstrap an instance that is configured as static IP through config drive, instead of the generic DHCP.  So with nova boot, everything seems to be happy but the instance just failed to get the static IP for some reason. Doing a nova console-log, the IP address shows like this in cloud-init.

cloud-init[775]: Cloud-init v. 0.7.5 running 'init' at Fri, 11 Sep 2015 17:56:46 +0000. Up 29.44 seconds.
cloud-init[775]: ci-info: +++++++++++++++++++++++Net device info+++++++++++++++++++++++
cloud-init[775]: ci-info: +--------+------+-----------+-----------+-------------------+
cloud-init[775]: ci-info: | Device |  Up  |  Address  |    Mask   |     Hw-Address    |
cloud-init[775]: ci-info: +--------+------+-----------+-----------+-------------------+
cloud-init[775]: ci-info: |  lo:   | True | 127.0.0.1 | 255.0.0.0 |         .         |
cloud-init[775]: ci-info: | eth0:  | True |     .     |     .     | fa:16:3e:4d:35:34 |
cloud-init[775]: ci-info: +--------+------+-----------+-----------+-------------------+

Obviously the eth0 is not getting the static IP. Once again I mounted the config-drive and then try to find the /openstack/content/0000 file but I can't find it.

[root@nova ~]# find /mnt/openstack | grep content
[root@nova ~]#

This is pretty weird as I remember the file should be there to allow static ip assignment to work through config-drive. So taking further looks, it seems the problem go to the subnet configuration. The subnet that the instance is on is having "enable_dhcp" equal to true, and that prohibited the config-drive to create the openstack/content/0000 file.

To disable DHCP for a subnet, run this.

# neutron  subnet-update  $SUBNET_UUID --enable-dhcp=False