I did a quick fix that solved this:
- Remove kie-server WAR file from tomcat webapps directory, so that it won't undo the next step.
- Delete the problem file webapps/kie-server-6.3.0.CR2-ee7/WEB-INF/lib/slf4j-api-1.7.2.jar
I did also the following which might be needed:
- I replaced latest slf4j jar files from /opt/apache-tomcat-8.0.26/lib directory with version 1.7.2 files. Perhaps Drools trusts in that it should work with latest version as well since they don't ship the files needed for deployment with the WAR files, but ask everyone to fetch latest files. So this step might not be necessary.