Action type aware scheduling

Currently in FilterScheduler there is no variable indicating which action triggered the request that is currently processed. The behavior of filters and weighers may be different depending on the fact if currently processed request is scheduling/rescheduling/migration/resize/live-migration/evacuation/unshelving.

What this blueprint proposes is to add a new variable which would keep an action type. It should be accessible from custom scheduler filters and weighers. This could be accomplished using filter_properties dictionary - a new key like 'action_type' can be added there.