Decouple networking-dynamic-routing from neutron via neutron-lib

This is a continuation of the neutron-lib saga.

While our initial work in [1] laid a foundation for neutron-lib's future by plumbing out the approaches/processes/implementations/etc., as discussed at the Pike PTG we need additional neutron-lib BPs that are containable within a single release.

The new approach for tracking neutron-lib work is to create a BP for a target sub-project and use the BP track the rehoming of that sub-project's neutron dependencies to neutron-lib + consume them. The BP is closed once the sub-project it targets no longer depends on neutron, but neutron-lib instead.

This BP targets the networking-l2gw project and includes the work to:
- Rehome networking-ovn's neutron dependencies to neutron-lib (refactoring as needed).
- Release them via neutron-lib pypi package.
- Consume them in networking-l2gw, thereby removing the respective neutron dependency(s).

For details on the rehoming process, please see [2].

[1] https://blueprints.launchpad.net/neutron/+spec/neutron-lib
[2] https://docs.openstack.org/developer/neutron-lib/contributing.html