Fix and unify setting up of virtual environment

It's aimed to allow setting up the virtual environment on Juwels, Juwels Booster and HDFML with one single script. The working branch collects many adaptions from #31 (closed) and #49 (closed) , however the Horovod integration is postponed until the parallelization works properly. Note, that this script can be used with the container environment anyway, since Horovod is provided with the TensorFlow container image!