Defines a common base class, NeutronApiTranslator.

Defines NeutronApiTranslator, a common abstract base class for
concrete Neutron model translatior implementation such as
NetworkTranslator. Defines an API method toMido, which converts an
operation (i.e. CREATE, UPDATE, DELETE) on a high-level Neutron model
object into a list of operations on low-level MidoNet model objects.

Also defines TranslationException that wraps around low-level
exceptions such as ObjectExistsException thrown by Storage.
NeutronApiTranslators are expected to catch those low-level Storage
Exceptions and rethrow them wrapped in TranslationException. Their
clients are expected to catch this Exception class and perform some
exception handling.

Fix MN-2999

Change-Id: Ibfb8de2d65482fc045d017efb950769982447821