Move instance_resize code paths to BDM objects

Make both resize_instance and confirm_resize compute manager methods use
BDM objects. This is achieved by moving the helper method
_terminate_volume_connections to use BDM objects.
This patch also avoids unnecessary calls to conductor as now all helper
methods used allow for passing the already dug up BDM list.

Part of blueprint: icehouse-objects
Part of blueprint: clean-up-legacy-block-device-mapping

Change-Id: Idf370609e23aff45e18a67c76768ba80511c2801
Merge "Move instance_resize code paths to BDM objects"