ceph: make -h/--help show match when some args are supplied

ceph: make -h/--help show match when some args are supplied

Currently,

 # ceph orch ls -h
 ...
 orch ls [<service_type>] [<service_name>] [--export] [--  List services known to orchestrator
  format {plain|json|json-pretty|yaml}] [--refresh]
 # ceph orch ls osd -h
 ... nothing ...

because the CLI is provided more arguments than the command prefix.  Make
-h drop right-hand args until we get at least one prefix match.  This
means we can have a partial command written with some args and add -h to
get a usage for that command.

Signed-off-by: Sage Weil <sage@newdream.net>