south-frr: south daemon that controls FRR

[ishidawataru]

FRR: https://github.com/FRRouting/frr

First, investigate if the Sysrepo northbound plugin works as expected.

It looks like the plugin supports the following YANG models.

https://github.com/FRRouting/frr/tree/master/yang

If this is the case, south-frr would work as a translator between FRR YANG models and Goldstone YANG models.

Since Goldstone doesn't have YANG models for L3 routing yet, we might take FRR YANG models mostly as is by just changing the model name and prefixes.

We also need to consider if using the same sysrepo instance between Goldstone and FRR is a good idea.

If we use the same sysrepo instance, FRR YANG models will become visible to Goldstone north daemons.
However, north daemons should not configure FRR directly with FRR YANG models. They should always use Goldstone YANG models and let south-frr do the translation.

To make the FRR YANG models invisible from the north daemons, we might create a dedicated sysrepo instance for FRR and let south-frr bridge two sysrepo instances.