filter: add per-aggregate filter to configure max_instances_per_host

Adds a filter AggregateIoOpsFilter which provides the ability
to read from aggregates metadata "max_instances_per_host".

DocImpact
Implements: blueprint per-aggregate-max-instances-per-host
Change-Id: I569aab7842d2c67042b89c75941a4664a599a7f6
Merge "filter: add per-aggregate filter to configure max_instances_per_host"