avoiding NPE when installing/stopping tomcat/tomee-webapp

git-svn-id: https://svn.apache.org/repos/asf/tomee/tomee/trunk@1458767 13f79535-47bb-0310-9956-ffa450edef68
ensuring we use the correct version of tomee-webapp

git-svn-id: https://svn.apache.org/repos/asf/tomee/tomee/trunk@1458768 13f79535-47bb-0310-9956-ffa450edef68