[Minor] Fix the value represented by spark.executor.id for consistency.

The property  `spark.executor.id` can represent both `driver` and `<driver>`  for one driver.
It's inconsistent.

This issue is minor so I didn't file this in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3812 from sarutak/fix-driver-identifier and squashes the following commits:

d885498 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into fix-driver-identifier
4275663 [Kousuke Saruta] Fixed the value represented by spark.executor.id of local mode