optionally specify TCP port for Jetty HTTP server in console

In org.apache.edgent.console.server.HttpServer, the server port is hardwired to "0", so Jetty chooses a random port number each time it starts. This is a problem when Edgent runs in a Docker container. By default, Docker blocks access to all ports in a container. Port access can be enabled for a specific port by number when a container is created, but not after it has started running.

Perhaps Edgent could honor a port number if specified with "java -Djetty.http.port=8080 ...", or use "0" if not.