tools: ynl: rename ops_list -> msg_list

ops_list contains all the operations, but the main iteration use
case is to walk only ops which define attrs. Rename ops_list to
msg_list, because now it looks like the contents are the same,
just the format is different. While at it convert from tuple
to just keys, none of the users care about the name of the op.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>