Add tests for instance upgrade

This is very important test scenario, but to make it work we need to:

• make entire test suite idempotent
• create scenarios for managing docker environemnt
• make it possible to manage docker environment from within test harness
• create gitlab upgrade test scenario:
  • spin up gitlab/gitlab-ce:latest
  • bind data volume to temporary directory
  • run all instance tests against the latest container
  • stop the old container
  • spin up gitlab/gitlab-ce:nightly
  • run all instance tests against nightly container
  • verify that the version has changed
• add CI job for GitLab CE
• add CI job for GitLab EE
• prepare PoC with broken docker image
  • broken migration