Add retry on exception to mattermost login

qa/qa/specs/features/browser_ui/1_manage/login/log_into_mattermost_via_gitlab_spec.rb

@@ -7,6 +7,7 @@ module QA
         Runtime::Browser.visit(:gitlab, Page::Main::Login)
         Page::Main::Login.perform(&:sign_in_using_credentials)
 
+        Support::Retrier.retry_on_exception do
           Runtime::Browser.visit(:mattermost, Page::Mattermost::Login)
           Page::Mattermost::Login.perform(&:sign_in_using_oauth)
 
@@ -16,4 +17,5 @@ module QA
         end
       end
     end
+  end
 end