Re-prioritize test that run during blackout and after the blackout

Per our discussion on May 23rd 2018 during the test plan execution, we need to better prioritize test during these 2 windows.

• During blackout - high-impact tests, decisions to be made if we need to rollback the infrastructure
• After blackout - issues that we can fix after we are on GCP infrastructure.

Below is the proposed order.

The detailed plan can be seen at: https://gitlab.com/gitlab-com/migration/blob/4d0ab31fe8b51a50febc6c2a6f60e2da107a43e9/.gitlab/issue_templates/test_plan.md