htools: implement post-alloc cluster status display

This patch changes the IAllocator result formatting workflow to return
the final node list, which can be then used to display the final node
status too—currently only the initial status can be shown, which is
only half useful.

Note that as the FIXME in the code says, doing this right for the
evacuate mode is hard; however, as that mode is deprecated, we can
live it for the moment.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>