tests: Add a topology that supports a large number of ecmp

tests: Add a topology that supports a large number of ecmp

Add a basic topology that allows the testing of BGP and zebra
at scale.  I built this to help me find and fix problems with
a large number of bgp peers.  Since I plan to keep using this
and as I understand it there are future plans to take this
higher, I would like to add this as a test that people can invoke
with this command:

sudo -E python3 -m pytest -s -vv --topology-only

Signed-off-by: Donald Sharp <sharpd@nvidia.com>