Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56717
Allow repeated Mapper.addHost() calls for the same host. E.g. if the host has been implicitly added by addContextVersion() call.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1610564 13f79535-47bb-0310-9956-ffa450edef68