support for parallel tests

https://github.com/grosser/parallel_tests for details

We need to add that erb to the final config file, it looks like inside erb you can not have <% or %> in a string or I could not make it work without syntax errors at least so I am doing that weird thing. 
I am not completely satisfied but I can live with it.

After this goes in, I'll do a different PR for kender to support creating and running in parallel