Run tests also with parity client

[LefterisJP]

Analysis

Current State

Currently all tests in the SP run only with geth.

Problem

We should also run them with parity since parity is also heavily utilized out there and has quite a few differences with geth which lead to bugs in the raiden implementation.

Solution

Either run all scenarios in the SP both with parity and geth (may be a bit too much)
or randomly assign a client and use it for the scenario. This way over a long period of time we get enough coverage of both clients.

[deepbrook]

I would actually run all of them against both clients - randomly selecting clients doesn't seem sensible. That way, every other day a test failure would pop up and disappear the next time the scenarios are run.

Running them against both clients isn't a lot of trouble to implement. I would, however, wait for the concurrent run feature to be implemented.