Set SPARK_PUBLIC_DNS in spark-env.sh (to address SPARK-833)

This is intended to address [SPARK-833](https://spark-project.atlassian.net/browse/SPARK-833), an issue where Spark web UIs might be bound to non-public addresses when running on EC2.  We had previously solved this problem for the Master and Worker UIs, but `SPARK_PUBLIC_DNS` needs to be set in the driver's environment in order for its application web UI to bind to the correct public address.