mock's return value for processutils.ssh_execute

In tests/drivers/test_ssh.py, SSHPrivateMethodsTestCase tests are mocking
processutils.ssh_execute() and sets its return value to a list. However,
processutils.ssh_execute() doesn't return a list, it returns a tuple: (stdout, stderr).

This fixes the tests, by changing the return values to (stdout, stderr) tuples.

Change-Id: If146d80ad9acd33bd5c676e6b2ba2142c9b07a84
Closes-Bug: #1273452
Merge "mock's return value for processutils.ssh_execute"