Added capability to use the FF fake device.

With https://hg.mozilla.org/mozilla-central/rev/cb7eb7fcf67b, Firefox now supports fake devices. They didn't implement it as a flag but as constraint though. One needs to pass {audio: ..., video: ..., fake: true} to trigger the fake device. I need to add some code to make this possible; unfortunately it doesn't work to pass the fake constraint to the audio or video dicts, in which case I could have used the existing query parameters for audio and video in apprtc.

The motivation for this is to make it possible to run the Chrome-Firefox interop test on bots without real webcams.