Decouple Geo node identity from external_url

Previously, you could not use the same external_url for more than one Geo node, since external_url was used to identify Geo nodes.

This change adds the gitlab_rails['geo_node_name'] setting as an optional identifier if you don't want to identify Geo nodes by their external_url.

It defaults to nil here. GitLab Rails can do the defaulting to external_url.

This enables at least the following use-cases:

• All secondaries can be accessed via a geographically-aware load balancer
• Active-passive Geo nodes (if we implement secondaries as transparent primaries)

Needed for Rails MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10755