lvconvert: tune cache-pool zeroing

lvconvert: tune cache-pool zeroing

Follow-up to 27a767d5e8cedf9cac31eb3562cf8fdd4aa88b7c.
Tunning behavior in a way we always prompt when option --zero is NOT specified.
Without -Z lvm expects user wants to 'reset' cache-pool metadata
(they could have been splitted from some cached LV)

If user doesn't want to zero metadata he needs to specify -Zn.

User may also avoid prompting for zeroing by using -Zy for
cache-pool (basically equals using --yes  without -Z being given)
(unlike full convert case, there is no cache-pool being converted,
so there is not 'uncoditional' prompt in this case).