Allow the global naming context to be reset. Useful when running multiple embedded instances in series since it allows each instance to configure its own global naming context.

      Doesn't help with multiple instances in parallel - but that was broken anyway.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@907727 13f79535-47bb-0310-9956-ffa450edef68