Create unit test and coverage report with pytest

tower-cli easily allows to:
- pass extra variables to the job
- wait for the job to complete

Deploy to staging on tags as well.

Note that the Ansible role shall use the ":master" tag for the image

- Use gitlab-ci services instead of docker-compose to run postgres and
  redis
- Switch to docker socket

Every tagged release is automatically deployed to staging
by launching the job template from Ansible AWX

CI_COMMIT_REF_NAME is the branch or tag name
1. When pushing a tag the TEST_IMAGE doesn't exist. We always pull master
for cache.
2. Make sure the pull of the cache image doesn't fail even if the image
doesn't exist