[SPARK-21456][MESOS] Make the driver failover_timeout configurable

## What changes were proposed in this pull request?

Current behavior: in Mesos cluster mode, the driver failover_timeout is set to zero. If the driver temporarily loses connectivity with the Mesos master, the framework will be torn down and all executors killed.

Proposed change: make the failover_timeout configurable via a new option, spark.mesos.driver.failoverTimeout. The default value is still zero.

Note: with non-zero failover_timeout, an explicit teardown is needed in some cases. This is captured in https://issues.apache.org/jira/browse/SPARK-21458

## How was this patch tested?

Added a unit test to make sure the config option is set while creating the scheduler driver.

Ran an integration test with mesosphere/spark showing that with a non-zero failover_timeout the Spark job finishes after a driver is disconnected from the master.

Author: Susan X. Huynh <xhuynh@mesosphere.com>

Closes #18674 from susanxhuynh/sh-mesos-failover-timeout.