Use unstable contracts env when running SP

[karlb]

raiden-network/raiden#6862 allows us to select which contracts to use for SP run. We don't want to use the demo contracts, which are the default. Instead we want to use the "unstable" contracts environment by calling Raiden with --development-environment unstable.

Should we add this as an optional entry to the SP environment files and set it to unstable in https://github.com/raiden-network/scenario-player/blob/master/scenario_player/environment/development.json ?