Unit tests improvements

- From now on, black must pass. Jenkins will give a -1 if it doesn't
- Ignore E203 because it is not pep8 compliant, and conflicts with black
    https://github.com/psf/black/issues/315#issuecomment-395457972
- Black fixes and minor fix in unit test
- Fix devops-stages/stage-test.sh: jenkins was not giving a -1 when tox
was failing

Change-Id: Ic7318dcb6d4006a271607cd064a1f36d0936b7b9
Signed-off-by: David Garcia <david.garcia@canonical.com>