Modifications to CentOS build

Hello,

These are some modifications I made to the CentOS build components in order to satisfy some requirements I had. Most of these requirements involve ensuring snapshots are correctly created:

1. udev must not add any rules about the virtual NIC or snapshots will be launched with an incorrect NIC, such as eth1 when it should be eth0.
2. Resizing of the root partition must happen on each boot so a snapshot can resize itself.

Both of these can be manually addressed by the user post-image deployment, but I feel that's not usually the user's responsibility.

One final change I made was to swap out the fdisk portion of the root partition resizing with growpart.

Please let me know if you have any questions, comments, or suggestions to achieve these goals better. I don't expect this pull to be merged -- this is more to just share the changes I made.

Thanks,
Joe