Create dissect-image action to extract separate filesystem images

[obbardc]

We need a solution to extract sparse images from a created disk image.

• input is a list of partition name and filename
• output is a number of sparse images in $ARTIFACTDIR
• action could be defined anywhere in the recipe, but run after the fakemachine has finished (e.g. after the image has been unmounted) and before the postprocess: true commands.