Expand and clarify Geo DR instructions

Improvements to the Geo Disaster Recovery doc: https://docs.gitlab.com/ee/gitlab-geo/disaster-recovery.html

  • Add instructions for taking down primary if it isn’t completely down
  • ‘Log-in’ is ambiguous in Log-in to your secondary node with.... Explicitly state SSH
  • Specify exactly what gitlab.rb changes are needed to apply to the secondary in order to make it a primary
    • Determine if it matters if geo_primary_role['enable'] = false. If it must be true, then we need the trust_auth_cidr_addresses config as well.
    • Decide if we should store a replica of the primary gitlab.rb on the secondary. If so, add instructions during Geo setup (or create an issue to make this happen) - moved to: https://gitlab.com/gitlab-org/gitlab-ee/issues/4421
  • Once DNS change to primary has propagated, update URL in gitlab.rb to reflect.

Other Geo DR instruction-related issues that deserved their own issue:

Edited by James Ramsay (ex-GitLab)