Implement IAllocator multi-evacuate mode

This is a new mode that request a solution for the evacuation of
multiple nodes. The external script will be fed a list of names, and is
expected to return a list of [instance, new_node(s)] lists, detailing
the evacuation path of each instance.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>