Start workerui on public addr

1.first the ip parameter in JettyUtils is actually not used, I removed that in JettyUtils.startJettyServer

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$ 

 I changed that start-master and start-slave, so that they always export SPARK_PUBLIC_DNS (when the user didn't set it explicitly)

3.the address to get public dns name in AWS is changed