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 "SSHPower driver raises IronicExceptions"