Use ec2-metadata in start-slave.sh to detect if running on EC2

This should finally fix [SPARK-613](https://spark-project.atlassian.net/browse/SPARK-613), the issue where the standalone web UI would link to internal addresses.

It turns out that #419 applied the same change, but only to `start-master.sh`, so some workers were still starting their web UI's using internal addresses.

After merging, please cherry-pick this into branch-0.7.