ceph_argparse: introduce CephBool arguments

ceph_argparse: introduce CephBool arguments

This replaces use of CephChoices for cases like
--yes-i-really-really-mean-it.

It's a relatively invasive change, because both
client and server need to handle backward compatibility:
 - Clients are easy, they just cope with the server's
   use of CephChoices/CephString with a few special case
   strings to recognise --yes-i-really-mean-it and similar
 - Servers are harder, they have to convert CephBool arguments
   into a similar CephChoices argument that older clients will
   understand.  This involves propagating feature flags into
   some code paths that couldn't see them before.

Signed-off-by: John Spray <john.spray@redhat.com>