buildbot: better place vor worker-vars

[Akira25]

currently, the workernames and other things are defined in the group_vars/all file. The idea of doing this was, to have a mutual data basis which gets used by buildbot-master and the workers in common (having only one place, where to maintain the data).

Having it in that file is a bit counter intuitive, as the data only applies for the buildbot and buildbotworkers groups. We should go for a solution, which is more concise about this, as discussed in #62 (comment)