ML2: DB changes for hierarchical port binding

To support hierarchical port binding, the driver and segment columns
are moved from the ml2_port_bindings and ml2_dvr_port_bindings tables
to a new ml2_port_binding_levels table. This new table can store
multiple levels of binding information for each port. It has the host
as part of its primary key so that it can be used for both normal and
DVR port bindings.

The cap_port_filter column is also removed from the
ml2_dvr_port_bindings table, since the adjacent driver and segment
columns are being moved, and this can trivially be done via the same
DB migration. It was included in the table by mistake and was never
used.

The logic required for hierarchical port binding will be implemented
in a dependent patch.

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

Change-Id: I36c460813bd6826415f0fd43ae5d89eb5a6cf55f
Partially-implements: blueprint ml2-hierarchical-port-binding