ROCKS7: Update cluster-kickstart-pxe and related for CentOS7 and grub2

### Attempt to use cluster-kickstart-pxe to PXE boot a node fails
```
# ssh vm-vct03-00 /boot/kickstart/cluster-kickstart-pxe
/boot/kickstart/cluster-kickstart-pxe: line 5: /etc/rc.d/init.d/rocks-grub: No such file or directory
/boot/kickstart/cluster-kickstart-pxe: line 10: /boot/kickstart/cluster-kickstart: No such file or directory
```