Start workerui on public addr

1. first the ip parameter in JettyUtils is actually not used, I removed that JettyUtils.startJettyServer(port, handlers)

2. the assumption that the ec2 instance hostname always starts with ec2-internal is no longer right

e.g. ubuntu@ip-172-31-32-76:~/spark-0.8.1$ 

3. the address to get public dns name in aws is changed

for 2, I changed that start-master and start-slave, so that they always export SPARK_PUBLIC_DNS (when the user didn't set it explicitly)