Moving CRUD DB operation for objects from db/api.py to objects/db/api.py
Renaming object get_by_id(id) to get_object(**kwargs)
Many models in Neutron DB have complex primary keys, concatenated from
a few properties. This patch adds ability to define multiple primary keys
in NeutronDbObject, which are automatically evaluated into DB query
when performing operations.
Partial-Bug: #1541928
Change-Id: I0f63a62418db76415ddd40c30c778ff7541b93dc