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Break out image creation via diskimage-builder into a separate role. Also break out create-bootable-image for consistency, though it is legacy stuff that we do not expect to use in future. In addition to the above, clean up a couple of typos, etc. in the adjacent code and documentation. Implements: blueprint bifrost-deployment-image Change-Id: If3dbfa6a6b4e3f6eee9b614342dbf5bccadfbd65