networking guide: added info about path_mtu option
networking guide: added info about path_mtu option

This option can be used for ML2 plugin to specify MTU for tunneled
networks that is different from the global one, similar to
physical_network_mtus for VLAN/flat networking.

Change-Id: I34a0a74d734ee8b7850b6aed75f7b066f13b2bb8
Related-Bug: #1552089
Related-Bug: #1454328

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals