Build automated integration tests for GitLab Geo

I think we'll want to do something similar with GitLab QA and start with a number of Docker images that test a number of scenarios with a Geo primary and secondary:

Things to test on both primary and secondary:

• Creating a new project works
• Git push
• Git push w/ LFS
• Issue Attachments
• Avatars
• etc.

@grzesiek We may need your time to get going on this as part of making Geo GA.

/cc: @ayufan