Add the iptables rule to allow gre traffic

Currently there's no rule to accept gre packets which are required for the
Neutron GRE networking case, so they are dropped due to the '999 drop all' rule

Change-Id: Ia79b6e8b26a8ccf7846affbd2dbf6aa758ac4947
Closes-bug: #1355794
Merge "Add the iptables rule to allow gre traffic" into stable/5.0