Add callbacks-based system to Neutron

Add callbacks-based system to Neutron

This system helps Neutron entities to cooperate in a loose manner.

This is especially important in face of the advanced service split, where each
service gets to live independently from the core. This is also useful to clean
up some of the tight coupling between ML2 and L3, or L3 and VPN/FW.

This work was touted to be part of the effort on the v3 plugin API design.
(aka perestroika), however that is not going to bear the expected fruits
in time for the Kilo release. This framework is instead pretty crucial to
cleaning up the split between the various Neutron components and some of
the coupling between ML2 and DVR.

Subsequent patches will show how this framework is put into practice in relation
to cleaning up/decoupling the various Neutron components, however, a devref
how-to is added in the context of this patch to further clarify how the proposed
mechanism works and can be used.

Related-blueprint: services-split
Related-blueprint: plugin-interface-perestroika

Change-Id: I498aeb0773822707e82763f1f0022c580308bde0