Since the issue is caused by cloudinit package and nor related to GRE nor Neutron, I'd consider renaming it to "CentOS 6 cloud image with cloud-init-0.7.5 can not retrieve OpenStack metadata"
Since the issue is caused by cloudinit package and nor related to GRE nor Neutron, I'd consider renaming it to "CentOS 6 cloud image with cloud-init-0.7.5 can not retrieve OpenStack metadata"