Scheduler: Adds per-aggregate filters

The aim of this spec is to implement 3 new filters for the
scheduler to provide the ability to set options per-aggreate.

These blueprints was already approved for icehouse.
blueprint per-aggregate-disk-allocation-ratio
blueprint per-aggregate-max-instances-per-host
blueprint per-aggregate-max-io-ops-per-host

Change-Id: I2b278e0765b6ff4df137f726fa83ce3ac4060ef5
Merge "Scheduler: Adds per-aggregate filters"