Allow the gre protocol on overcloud

On Fedora the gre protocol needs to be allowed though iptables in order
for the overcloud to be able to user gre tunnels. This may not be required
in all cases (some have reported not needing it) but it is definitely
required in any setup I have tried.

Change-Id: I1613706ab19d8ab78a69ce271e8ae70c09bfd1b2
Merge "Allow the gre protocol on overcloud"