JIRA INFRA-2832

- add term coverage report to capture it in gitlab-ci
- add pipeline and coverage badges

Overwrite the default command to pass discovery.type=single-node
Environment variable with "." can't be exported from bash.

See https://gitlab.com/gitlab-org/gitlab-runner/issues/2410

Version shall be identical to the one used in production: 6.4.2

JIRA INFRA-575 #action In Progress

JIRA INFRA-526

New feature added in GitLab 11.2.
See https://docs.gitlab.com/ee/ci/yaml/README.html#artifacts-reports-junit

- requirements-to-freeze.txt is used to list only the needed packages
- requirements.txt is used to build a reproducible environment
- requirements-dev.txt is for testing and building the documentation

torn was configured to run playbooks on the GPN via a proxy.

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