Remove compute API get_instance_bdms method

This method is just an unnecessary level of indirecton, now that we have
BDM objects that give us a nice API on top of db calls, so we can remove
it,

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

Change-Id: I02226cfb67955b604d6bed513211ef46fb65dc69
Merge "Remove compute API get_instance_bdms method"