Move detach_volume and remove_vol_connection to BDM objects

This patch moves detach volume code paths to use BDM objects. Since both
detach_volume and remove_volume_connection compute manager methods,
since both of them use the _detach_volume method that needed to be
converted as well.

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

Change-Id: I1e9af9be0ea318f568180ddbd6cd0a25e8bb0bfd