SPARK-5403: Ignore UserKnownHostsFile in SSH calls

See https://issues.apache.org/jira/browse/SPARK-5403

Author: Grzegorz Dubicki <grzegorz.dubicki@gmail.com>

Closes #4196 from grzegorz-dubicki/SPARK-5403 and squashes the following commits:

a7d863f [Grzegorz Dubicki] Resolve start command hanging issue