Rename list-images to list.

[supakeen]

More general naming shenanigans. Since all of our subcommands generally will work on images I see no good reason to have this command contain images in the name. If we want to have non-image commands we have a few options:

We could have:

1. image-builder build [image-type]
2. image-builder describe [image-type] (see cmd: add new describe-image command #9).
3. image-builder list

Or alternatively we could have -image in all of them:

1. image-builder build-image [image-type]
2. image-builder describe-image [image-type]
3. image-builder list-image

Or alternatively they can all exist under an image subcommand:

1. image-builder image build [image-type]
2. image-builder image describe [image-type]
3. image-builder image list

Which one do you prefer?