Master broken for testing simultaneous clusters

[cbaenziger]

In #1304 we lost the ability from #788 to build multiple clusters at a time. This breaks developers testing master and a development cluster simultaneously and other workflows.

[aespinosa]

Looking at the code, #788 modified cluster.txt after copying it from stub-environment.

To enable it again, it's probably best to do it in https://github.com/bloomberg/chef-bach/blob/master/tests/automated_install.sh#L43