Fix grub.cfg to not use consistent networking naming feature

RHEL7 supports the 'Consistent Network Naming' feature, that names devices after their physical location.  However, in a VM, this doesn't apply.  Furthermore, these packer scripts don't allow for it, since the udev rules are already disabled. This fix implements the net.ifnames=0 option, which is required by RHEL7.

This also fixes a bug where using the vmware-vmx builder with the VMX created by these templates will not work, since the network interface never comes up.