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

ec2/spark_ec2.py

@@ -899,6 +899,7 @@ def stringify_command(parts):
 
 def ssh_args(opts):
     parts = ['-o', 'StrictHostKeyChecking=no']
+    parts += ['-o', 'UserKnownHostsFile=/dev/null']
     if opts.identity_file is not None:
         parts += ['-i', opts.identity_file]
     return parts