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.
Merge pull request #1 from sjmh/networknaming

Fix grub.cfg to not use consistent networking naming feature