This PR adds the URLs to the driver logs to `SparkListenerApplicationStarted` event, which is later used by the `ExecutorsListener` to populate the URLs to the driver logs in its own state. This info is then used when the UI is rendered to display links to the logs.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #6166 from harishreedharan/am-log-link and squashes the following commits:

943fc4f [Hari Shreedharan] Merge remote-tracking branch 'asf/master' into am-log-link
9e5c04b [Hari Shreedharan] Merge remote-tracking branch 'asf/master' into am-log-link
b3f9b9d [Hari Shreedharan] Updated comment based on feedback.
0840a95 [Hari Shreedharan] Move the result and sc.stop back to original location, minor import changes.
537a2f7 [Hari Shreedharan] Add test to ensure the log urls are populated and valid.
4033725 [Hari Shreedharan] Adding comments explaining how node reports are used to get the log urls.
6c5c285 [Hari Shreedharan] Import order.
346f4ea [Hari Shreedharan] Review feedback fixes.
629c1dc [Hari Shreedharan] Cleanup.
99fb1a3 [Hari Shreedharan] Send the log urls in App start event, to ensure that other listeners are not affected.
c0de336 [Hari Shreedharan] Ensure new unit test cleans up after itself.
50cdae3 [Hari Shreedharan] Added unit test, made the approach generic.
402e8e4 [Hari Shreedharan] Use `NodeReport` to get the URL for the logs. Also, make the environment variables generic so other cluster managers can use them as well.
1cf338f [Hari Shreedharan] [SPARK-7657][YARN] Add driver link in application UI, in cluster mode.