Draft: Let Clickable install pip modules

This MR is trying to address #354

How to test

• changes to clickable.yaml:

install_pip:
  - jsonschema==3
pip_install_dir: vendor

dependencies_host:
- python3-pip
- python3-setuptools

• changes to Python code:

import sys

sys.path.append("vendor")

import jsonschema

Issues

• the pip install command inside the container does fail if not executed with root permissions
• The __init__.py of at least jsonschema includes pkg_resources which tries to access paths it is not allowed to, such as /home/phablet/.local/lib/python3.5/site-packages.

ToDos

• Evaluate the use of venv to fix system path accesses
• Make pip3 install work without root permissions
• Bring pip and setuptools into docker images
• Adapt app template accordingly