Update centos-6-openstack.ks

a couple of small fixes:

- enable console-logging in the way Nova expects (i.e. make `nova console-log $VM` work; see [here](https://ask.openstack.org/question/108/configuring-image-to-write-to-console-log/) for more information)
- add `ec2-user` for `cloud-init` (the user that gets the SSH key injected in the default config)
- add `NOZEROCONF` variable that prevents CentOS from adding a 169.254.0.0-route that troubles fetching metadata (at least with Quantum, but Zeroconf's probably not needed in a cloud VM anyways)