Minimize CE/EE difference in Gitlab::Auth::Saml::Config

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/auth/saml/config.rb

@@ -14,6 +14,10 @@ module Gitlab
           def external_groups
             options[:external_groups]
           end
+
+          def admin_groups
+            options[:admin_groups]
+          end
         end
       end
     end