SPARK-2450 Adds executor log links to Web UI

Adds links to stderr/stdout in the executor tab of the webUI for:
1) Standalone
2) Yarn client
3) Yarn cluster

This tries to add the log url support in a general way so as to make it easy to add support for all the
cluster managers. This is done by using environment variables to pass to the executor the log urls. The
SPARK_LOG_URL_ prefix is used and so additional logs besides stderr/stdout can also be added.

To propagate this information to the UI we use the onExecutorAdded spark listener event.

Although this commit doesn't add log urls when running on a mesos cluster, it should be possible to add using the same mechanism.

Author: Kostas Sakellis <kostas@cloudera.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #3486 from ksakellis/kostas-spark-2450 and squashes the following commits:

d190936 [Josh Rosen] Fix a few minor style / formatting nits. Reset listener after each test Don't null listener out at end of main().
8673fe1 [Kostas Sakellis] CR feedback. Hide the log column if there are no logs available
5bf6952 [Kostas Sakellis] [SPARK-2450] [CORE] Adds exeuctor log links to Web UI

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