Extend image parsing to accept qemu-style booleans. Introduce "cloudinit" Image argument.

--image=ubuntu64,foo,-bar will pass foo=True, bar=False as kwargs
to the Image constructor.

Add a minuscule amount of checking to the args for --image.

Introuduce "cloudinit" as an Image boolean.

Added unit tests for all the image parsing. Badly needed.

Change-Id: I4402752f6512a5e1953e83202f5b744d2e75bc12