Tests/improvements

Created by: yoptar

• remove env.CUDA_VISIBLE_DEVICES=0 from Jenkinsfile and rely on node's variables
• allow to set api port with DP_PYTEST_API_PORT env. variable
• store downloads cache inside the repo dir that will always be cleaned instead of /tmp/
• allow to disable downloads caching with setting DP_PYTEST_NO_CACHE env. variable
• allow adding testing parameters pytest_* anywhere in a config, not just inside a train block