Use Spark public DNS if available to report Flow UI
If spark env variable `SPARK_PUBLIC_DNS` is defined, then we should use it to report Flow UI address.
If I understand it correctly, we want to hide ip address h2o cluster is using and in case public dns name is set, use that. See the tiny commit if this is what we're looking for
This therefore also implements https://0xdata.atlassian.net/projects/SW/issues/SW-338?filter=allopenissues