BZ#2107509 [RFE][OVN] Relay metadata connections to a 2-tier SB ovsdb-server

Right now, each compute node contributes with 2 connections to the Southbound database. In order to scale better, RHOSP could benefit from a 2-tier deployment where the metadata connections (mostly read-only) can be relayed to.

All the necessary changes are present in OVS 2.17 and this RFE would essentially involve changes in RHOSP deployment and configuration at TripleO level.

For more information please check: https://docs.openvswitch.org/en/latest/topics/ovsdb-relay/