Merge/reference Trellis tutorial from main stratum repo

[ccascone]

The Trellis tutorial in this repo seems to overlap with the one provided in the main stratum repo: https://github.com/stratum/stratum/tree/master/tools/mininet/examples/trellis

Moreover, this one feels confusing for anyone who doesn't know what Trellis is or how to use ONOS. The one in the Stratum repo instead provides a bit more context and commands to test connectivity as well as to use the ONOS CLI.

I suggest to either reference the one from the main repo here, or to move its content in this repo.