filter: add per-aggregate filter to configure disk_allocation_ratio

Adds a filter AggregateDiskFilter which provides the ability to
read from aggregates metadata the "disk_allocation_ratio".

DocImpact
Implements: blueprint per-aggregate-disk-allocation-ratio
Change-Id: I79d59fcdfb09e67ed6f12113615c673624b24a19
(cherry-picked from commit 56a9512f1443c8df3cfb0153d358c8cd8da50336)

Upstream-Review: https://review.openstack.org/#/c/65452/
Closes-rally-bug: DE767
Backported-from: juno