Implements ApiTranslator for Neutron Network.

- NeutronApiTranslator is a common base class for ApiTranslator
  implementations for Neutron model.
- NetworkTranslator translates operations on a Neutron Network into
  corresponding operations on MidoNet Network.

Fix MN-2999

Change-Id: I4d587887150f5ebff3d8defcca0330545dee700c
Merge "Implements ApiTranslator for Neutron Network."