Use `per wifi` instead of `per ap`.

One access point can have many wifi networks, we only deal with networks,
not physical access points.
Use a `00` prefix for wifi key. `ab` is an ad-hoc network.

While we don't treat ad-hoc networks in special ways now, we might in the
future and the filtering test might continue to succeed based on ad-hoc
filtering rather than ingress filtering.
make flake8 happy
Use fewer dots in new heka metrics.

Each dot creates an additional sub-level in graphite reporting. I prefer
having to expand the stats tree less often :)