Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56248
Allow the deployer to update an existing WAR file without
undeploying the existing application if the update flag is set.
This allows any existing custom context.xml for the application
to be retained. To update an application and remove any existing
context.xml simply undeploy the old version of the application
before deploying the new version.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1577199 13f79535-47bb-0310-9956-ffa450edef68