Instead of actually downloading the logs, just verify that the logs link is actually
a URL and is in the expected format.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #6680 from harishreedharan/simplify-am-log-tests and squashes the following commits:

3183aeb [Hari Shreedharan] Remove check for hostname which can fail on machines with several hostnames. Removed some unused imports.
50d69a7 [Hari Shreedharan] [SPARK-8136][YARN] Fix flakiness in YarnClusterSuite.