Ask HN: Tools/infrastructure for running tests on multiple hosts simultaneously?

My workflow has me running tests, in an interactive and iterative way on up to 250 hosts at a time, with machines divided into one of a set of up to 6 roles (some may have multiple roles). I currently use GNU Parallel or Python Fabric depending on what I need to do but this requires writing a bunch of custom code for every set of tests I run. Is there anything else out there that could reduce the overhead of my approach?