Code review comment for ~peter-sabaini/charm-mongodb:upgrade-functests

I haven't actually tested this myself, but it looks good. It doesn't look like this provides parity with the Amulet suite (correct me if I'm wrong), but it looks like it does port us over to Python 3 and provides a start of a new Zaza-based test suite.

I'm +1 for this, but would like one more pair of eyes given the size of this.