Add option to clean all parts

[Sergio Costas]

Currently, executing a "snapcraft clean" will destroy any container used to build a snap. There is the option of using "snapcraft clean PART_NAME", and that will just clean that part and rebuild it and any part that depends on it. Unfortunately, there is no way of cleaning all the parts to trigger a full rebuild without deleting the container and having to pull all the snaps and packages needed to build the container.

So, it would be useful to add "snapcraft clean --parts-only" or "snapcraft clean --all-parts" to just clean all the parts but without destroying the container.