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
SSHPower driver raises IronicExceptions

Modified the methods of SSHPower driver to only raise IronicExceptions
(instead of non-IronicExceptions). Specifically, an SSHCommandFailed
exception is raised instead of an exception raised by
processutils.ssh_execute().

Also modified one unit test, to check for an InvalidParameterValue
exception instead of the more general IronicException.

Change-Id: I9bde8efa614f76b3ba950393cb49c5c619407aa8
Partial-Bug: #1267693
Merge "mock's return value for processutils.ssh_execute"
Merge "SSHPower driver raises IronicExceptions"