tests: pin python dependencies

tests: pin python dependencies

This commit ensures that the tests always run with the exact same
version for all the Python dependencies. It uses pip-tools to achieve
that.

Our top-level dependencies are specified in the requirements.txt file.
From this file, pip-tools generates the requirements-{2,3}.txt file,
which pins all the versions of all the recursive dependencies, and is
used to install the packages in the test virtualenv.

To change or add a top-level dependency, update requirements.txt and run
make test as usual with python2 and python3. The requirements-{2,3}.txt
file will be updated and you can verify that nothing breaks. Then add
all requirements* files in your commit.

To refresh the python packages (i.e. get new versions of the recursive
dependencies, or of the dependencies that are not pinned in
requirements.txt), just run:
PYTHON=python2.7 make test-refresh-deps
PYTHON=python3.6 make test-refresh-deps
and this will update the requirements-{2,3}.txt files. Ideally we
should run this after each release.

Type: make
Change-Id: Ic533de3d06ec4019ff38f5231208da6f1025bfc7
Signed-off-by: Aloys Augustin <aloaugus@cisco.com>