Permit passing SSH keys into the Ironic API

SSH keys are equivalent to passwords for the ssh driver, and having to
special case deployment metadata to push them out of band to a file is
awkward. This patch adds the ability to pass the key in directly which
is much easier (in some environments).

Change-Id: I4421c80cb3dc0c8aee614c684d113a7ab6db6779
Merge "Permit passing SSH keys into the Ironic API"