When openvswitch firewall driver is used, it is required to load
nf_conntrack_proto_gre kernel module to make GRE tunnels from VM to VM
working properly.
This patch adds such info in ovs firewall documentation as it should be
deployer decision to load or not load this module.
This patch also adds sanity check which checks if nf_conntrack_proto_gre
module is loaded or not, and can warn user when this module is not
loaded.
It also adds loading of this kernel module in neutron devstack plugin.
Change-Id: Ic97ca00c804f0a540ee0dc53d9e4e07bf8410869
Closes-Bug: #1828053
b8a18dc22a