dissect-image: rename verity flag booleans

dissect-image: rename verity flag booleans

Let's make the booleans indicating verity state a bit more descriptive.

Let's rename:

    can_verity → has_verity: because that's really what this about
    whether verity data is included in the image. Whether we actually
    can use it is a different story.

    verity → verity_ready: this one should tell us if we have everything
    need to actually set it up, hence explicitly say "ready to use" in
    the name.

No change in behaviour. Just a bit of renaming.