Configure OVN to emit "need to frag" packets in case of MTU mismatch

It is useful when external network's MTU is lower then internal
geneve networks.
Host kernel needs to be in version >= 5.2 for this option to work.
All Kolla supported host operating systems have higher kernel version.

Change-Id: Id64e99b07e2bb5e6c97b784f4ffedafc7e7de188

ansible/roles/neutron/templates/ml2_conf.ini.j2

@@ -40,4 +40,5 @@ ovn_nb_connection = {{ ovn_nb_connection }}
 ovn_sb_connection = {{ ovn_sb_connection }}
 ovn_metadata_enabled = True
 enable_distributed_floating_ip = {{ neutron_ovn_distributed_fip | bool }}
+ovn_emit_need_to_frag = True
 {% endif %}

releasenotes/notes/ovn-emit-need-to-frag-2d6695596042db18.yaml

+---
+upgrade:
+  - |
+    Enabled ``ovn_emit_need_to_frag`` setting by default. It is useful when
+    external network's MTU is lower then internal geneve networks.
+    Host kernel needs to be in version >= 5.2 for this option to work.
+    All Kolla supported host operating systems have higher kernel version.