[SPARK-8988] [YARN] Make sure driver log links appear in secure cluste…

…r mode.

The NodeReports API currently used does not work in secure mode since we do not get RM tokens. Instead this patch just uses environment vars exported by YARN to create the log links.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #7624 from harishreedharan/driver-logs-env and squashes the following commits:

7368c7e [Hari Shreedharan] [SPARK-8988][YARN] Make sure driver log links appear in secure cluster mode.