Create test harness behavior; refactor current harnesses

This is part of the RT RFC Refactoring (https://github.com/basho/riak_test/issues/667) effort:

- Create a harness behavior and have each harness implement it

Introduce test_harness behavior; refactor current harnesses (rtdev, rtssh, rt_cs_dev, rtperf) so that they implement it; introduce rt_harness module that resolves currently configured harness and calls correct module for callbacks.