open gre in addition to vxlan

This patch teaches quickstack::neutron about GRE.  It opens GRE access
anywhere that vxlan access was already permitted.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1072325

We don't have that in `quickstack::neutron::firewall::vxlan`.  This module is basically a duplicate of the existing `vlan.pp` manifest.