Make cells rpc bdm_update_or_create_at_top use BDM objects

We were sending SQLAlchemy objects previously, which is bad practice
since they work only with Rabbit rpc backend. Inter-cell RPC calls will
still use dicts.

Change-Id: I28ee5f914914b85be577291edef6c285f055be5a
Merge "Make cells rpc bdm_update_or_create_at_top use BDM objects"