Affinity instance slot reservation

[daniel.pawlik]

Currently ServerGroupAffinityFilter schedules all instances of a group on the same host, but it doesn't reserve any slots for future instances on the host.

Example:
max_instances_per_host=10
quota_server_group_members=5

When user_1 would spawns 3 instances with affinity server group policy, all the instances will be scheduled on the same host (host_A).
If user_2 also spawns instances and they are placed on host_A, quota of max_instances_per_host will be reached, so user_1 can not add 2 new instances to the same server group and error "No valid host found" will be returned.

My proposition is to add new parameters to nova.conf to configure ServerGroupAffinityFilter:
- enable_slots_reservation (Boolean)
- reserved_slots_per_instance (-1 will count difference between max_instances_per_host and quota_server_group_members ; values bigger than 0 will reserve the indicated number of ports per group)

Nova scheduler checks if on a host there are any instances with affinity policy and based on that, it counts available slots.