Cisco nexus plugin port-binding table needs to be redesigned

[Baodong (Robert) Li]

In the current implementation, the table cisco_nexusport_bindings uses an automatically incremented field as its primary key.
cisco_nexusport_bindings:
    id = sa.Column(sa.Integer, primary_key=True, autoincrement=True)
    port_id = sa.Column(sa.String(255))
    vlan_id = sa.Column(sa.Integer, nullable=False)
    switch_ip = sa.Column(sa.String(255))
    instance_id = sa.Column(sa.String(255))

There should be one-to-one mapping between entries in the table and neutron ports. However, based on the above definition, such relationship is not properly maintained. Instead, neutron port id should be used as the table's primary key. Such change will improve the plugin design and code.

[Henry Gessau (gessau)]

Also, the vlan_id should not be nullable.
See the review comments here:
https://review.openstack.org/#/c/55411/6/neutron/plugins/cisco/db/nexus_models_v2.py