lock: drop machine-type default, but require for lock-many

Signed-off-by: Sage Weil <sage@inktank.com>
lock: make --summary list all machines by default

Signed-off-by: Sage Weil <sage@inktank.com>
lock: filter machine type for --list, --list-targets

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