Check if tracking DB is available when performing a Geo health check

lib/gitlab/geo/health_check.rb

@@ -3,6 +3,7 @@ module Gitlab
     class HealthCheck
       def self.perform_checks
         return '' unless Gitlab::Geo.secondary?
+        return 'The Geo database configuration file is missing.' unless Rails.configuration.respond_to?(:geo_database)
 
         database_version  = self.get_database_version.to_i
         migration_version = self.get_migration_version.to_i