[SPARK-13906] Ensure that there are at least 2 dispatcher threads.

## What changes were proposed in this pull request?

Force at least two dispatcher-event-loop threads. Since SparkDeploySchedulerBackend (in AppClient) calls askWithRetry to CoarseGrainedScheduler in the same process, there the driver needs at least two dispatcher threads to prevent the dispatcher thread from hanging.

## How was this patch tested?

Manual.

Author: Yonathan Randolph <yonathangmail.com>

Author: Yonathan Randolph <yonathan@liftigniter.com>

Closes #11728 from yonran/SPARK-13906.