[RFE] Adoption of "os-vif" in Neutron

[Rodolfo Alonso]

[Existing problem]
From `os-vif Nova SPEC`_, whenever a new Neutron mechanism driver is created, this results in the definition of a new VIF type. This situation generates two problems:

* Nova developers need to maintain the plug/unplug code in the VIF drivers, which is defined bt the needs of the Neutron mechanism.
* The format of the data passed between Nova and Neutron for the VIF port binding is fairly loosely defined (no versioning or formal definition).

"os-vif" is being adopted progressively in Nova. As said before, "os-vif" is in charge of the plugging and unplugging operations for the existing VIF types.

[Proposal]
To adopt "os-vif" project in Neutron, decoupling any plug/unplug operations from Neutron.

This RFE could be the container smaller contributions migrating step by step al VIF types in Neutron.

This topic will be discussed during the Denver PTG [2].

The proposed solution (to be discussed) is to add a new class in each mech driver agent, leveraging "os-vif" directly in our agent's plugging logic.

[Benefits]
* Centralize the plug/unplug operations in one common place.
* Provide to Neutron a way to define VIF types as required by Nova. The definitions contained in "os-vif" will be used by both projects.
* Remove from Neutron any plug/unplug driver specific operation, leaving to "os-vif" these actions. Neutron is supposed to be a L2-L4 SDN controller.

[References]
[1] `os-vif Nova SPEC`_: https://review.openstack.org/#/c/287090/
[2] https://etherpad.openstack.org/p/neutron-queens-ptg
[3] Nova os-vif library: https://review.openstack.org/#/q/topic:bp/os-vif-library,n,z