Per aggregate scheduling weight

Filter Scheduler uses the so-called weights during its work. A weigher is a way to select the best suitable host from a group of valid hosts by giving weights to all the hosts in the list.

In the current implementation, the weights are controlled by config options like
[filter_scheduler] cpu_weight_multiplier
the total weight of a compute node is calculated by combination of serveral weighers:
weight = w1_multiplier * norm(w1) + w2_multiplier * norm(w2) + ...

As it is controlled by config options, the weights are global across the whole deployment,
this is not convinient enough for operators and users.

This blueprint propose to allow set weights on host aggregates to have more flexibility
when scheduling.