[SPARK-17715][SCHEDULER] Make task launch logs DEBUG

## What changes were proposed in this pull request?

Ramp down the task launch logs from INFO to DEBUG. Task launches can happen orders of magnitude more than executor registration so it makes the logs easier to handle if they are different log levels. For larger jobs, there can be 100,000s of task launches which makes the driver log huge.

## How was this patch tested?

No tests, as this is a trivial change.

Author: Brian Cho <bcho@fb.com>

Closes #15290 from dafrista/ramp-down-task-logging.