Update iallocator design for node group-aware operations

A while ago a new ``multi-relocate`` mode was proposed and documented.
As it turned out, the interface had some deficiencies. With this patch
The relocation modes are reduced to two and split into separate
iallocator request modes: node-evacuate and change-group. Some request
and response requirements are clarified in the documentation.

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