Update the RAPI client for the migration mode

See the discussion on the previous patch about this. Basically unless we
want to a add a new 'feature' marking for the live migration parameter,
there is no simple way to handle this nicely in the client.

Given that the client was/is marked as experimental, this patch simply
replaces live with mode. This means that this client won't work with 2.1
clusters…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>