Update node group iallocator design to use job dependencies

While working on a function to submit jobsets, I realized that we
actually don't need them anymore. With the new job dependencies, the
iallocator plugin can just generate the right dependencies and gets the
same effect as with jobsets. This saves us from having to implement
jobsets in Ganeti core.

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