fix zap again
ceph-disk-prepare: enfoce disk vs dir/file for data, journal

Careful callers can do --data-dir or --journal-dev or similar to ensure
that we do what they want.  Without those args, we try to do the right
thing.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: only zap disk/partitinos if --zap-disk is provided

This is a change of behavior, but for the better, I think.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: allow journal_uuid to be predeclared

Signed-off-by: Sage Weil <sage@inktank.com>