Geo secondary node redirects to primary when OAuth provider is configured

Summary

When an OAuth provider is configured for the secondary node, it is not used. Instead, the user is still redirected to the primary node to authenticate.

Steps to reproduce

1. Configure an OAuth provider in gitlab.rb on the secondary node (I tested with Crowd)
2. Navigate to the URL of the secondary node. You are redirected to the primary node to authenticate

What is the current bug behavior?

When an OAuth provider is configured for the secondary node, it is not used. Instead, the user is still redirected to the primary node to authenticate.

What is the expected correct behavior?

You should authenticate with the OAuth provider configured in the secondary node. You should not be redirected to the primary node.

Possible fixes

Add a check here to see if an OAuth provider has been configured on the secondary node: https://gitlab.com/gitlab-org/gitlab/-/blob/v12.8.1-ee/ee/app/controllers/ee/sessions_controller.rb#L16

Customer ticket: https://gitlab.zendesk.com/agent/tickets/147539 (internal use)

Slack discussion: https://gitlab.slack.com/archives/C32LCGC1H/p1582913184133200