ML2: Driver API changes for hierarchical port binding

The following ML2 driver API changes are required to support
hierarchical port binding:

* Add segments_to_bind PortContext property containing the list of
  network segments with which a mechanism driver should try to bind
  the port. All mechanism drivers that bind ports should now use this
  property in place of network.network_segments, which contains only
  the network's static segments.

* Replace several PortContext properties (bound_segment, bound_driver,
  original_bound_segment, original_bound_driver) with new properties
  (binding_levels, top_bound_segment, bottom_bound_segment,
  original_binding_levels, original_top_bound_segment,
  original_bottom_bound_segment) in order to represent hierarchical
  bindings.

* Add stubbed-out continue_binding() method to PortContext, allowing
  mechanism drivers to partially bind the port.

All existing drivers and unit tests are updated accordingly.

The DB schema changes and logic required for hierarchical port binding
will be implemented in dependent patches.

Gerrit Spec: https://review.openstack.org/#/c/106185/

Partially-implements: blueprint ml2-hierarchical-port-binding

Change-Id: Icb1a016f4661e427cb6cfa3452802ba5e64b7124