Move _poll_volume_usage periodic task to BDM objects

This patch makes the _poll_volume_usage compute service periodic task
use BDM objects internally. It also moves a single instance query to
objects as a side effect. Also adds a missing test for the manager
helper method _get_host_volume_bdms.

Part of blueprint: icehouse-objects

Change-Id: I085fddfbe5872c0b2b044106c0d2507fd4ee32d8
Merge "Move _poll_volume_usage periodic task to BDM objects"