Small fixes to web UI:

- Use SPARK_PUBLIC_DNS environment variable if set (for EC2)
- Use a non-ephemeral port (3030 instead of 33000) by default
- Updated test to use non-ephemeral port too
Allow some wiggle room in UISuite port test and in EC2 ports
Merge pull request #849 from mateiz/web-fixes

Small fixes to web UI