Reproducible Python environment for e2e tests

Following up the work in MR !556 (merged) (issue #541 (closed)), the reproducibility of the e2e test environment should be improved.

Notes:

• Unpinned Python package are currently installed via the ansible playbooks and inside the ansible_image that's used in CI.
• Restructuring is needed, because all pip packages are installed into the same Python environment, which will underminde the benefits of pinning. Packages related to Ansible should not be installed into the same environment that krake and rok use.
• There seems to be confusion which Python packages are needed on the ansible control node (= inside the container executing ansible_image) and on the managed hosts, resulting in duplication and unnecessary installs/pollution.