[CONTINUUM-2713] improve login button detection

Works with existing web application, as well as after adjustments in new
Redback version needed for Struts 2.3.15.3

git-svn-id: https://svn.apache.org/repos/asf/continuum/branches/CONTINUUM-2713@1546719 13f79535-47bb-0310-9956-ffa450edef68