[SPARK-6175] Fix standalone executor log links when ephemeral ports or SPARK_PUBLIC_DNS are used

This patch fixes two issues with the executor log viewing links added in Spark 1.3.  In standalone mode, the log URLs might include a port value of 0 rather than the actual bound port of the UI, which broke the ability to view logs from workers whose web UIs had been configured to bind to ephemeral ports.  In addition, the URLs used workers' local hostnames instead of respecting SPARK_PUBLIC_DNS, which prevented this feature from working properly on Spark EC2 clusters because the links would point to internal DNS names instead of external ones.

I included tests for both of these bugs:

- We now browse to the URLs and verify that they point to the expected pages.
- To test SPARK_PUBLIC_DNS, I changed the code that reads the environment variable to do so via `SparkConf.getenv`, then used a custom SparkConf subclass to mock the environment variable (this pattern is used elsewhere in Spark's tests).

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4903 from JoshRosen/SPARK-6175 and squashes the following commits:

5577f41 [Josh Rosen] Remove println
cfec135 [Josh Rosen] Use webUi.boundPort and publicAddress in log links
27918c7 [Josh Rosen] Add failing unit tests for standalone log URL viewing
c250fbe [Josh Rosen] Respect SparkConf in local-cluster Workers.
422a2ef [Josh Rosen] Use conf.getenv to read SPARK_PUBLIC_DNS