Add SONiC support

[Chris L.]

Pybatfish supports SONiC per https://github.com/batfish/pybatfish/tree/master/docs/networks/sonic_example

The typical upstream example can be found at https://github.com/sonic-net/SONiC/blob/master/doc/config_db.json . If you'd like a more involved configuration I can happily anonymise my own and provide it.