Supports 'virtio-forwarder' vnic type for port

Supports the 'virtio-forwarder' vnic type for port sync with Neutron.
The 'virtio-forwarder' is an indirect mode for SR-IOV interfaces used to
connect the instances into the Neutron networks. It has been supported
since the Queens release:
I28315fb12f62dc7d52d099c211086b2b81eca39a

Note that 'virtio-forwarder' is already supported by Horizon:
openstack_dashboard/api/neutron.py:
  ('virtio-forwarder', _('Virtio Forwarder'))

Nova:
nova/network/model.py:
  VNIC_TYPE_VIRTIO_FORWARDER = 'virtio-forwarder'

openstackclient:
openstackclient/network/v2/port.py:
  choices=(...), virtio-forwarder

neutron-lib:
neutron_lib/api/definitions/portbindings.py
  VNIC_VIRTIO_FORWARDER = 'virtio-forwarder'

Task: 40435
Change-Id: Ie65f8858d6d1a1854702eb33c40ea827505bfe51

heat/engine/resources/openstack/neutron/port.py

@@ -283,7 +283,8 @@ class Port(neutron.NeutronResource):
               'the bindings extension.'),
             constraints=[
                 constraints.AllowedValues(['normal', 'direct', 'macvtap',
-                                           'direct-physical', 'baremetal']),
+                                           'direct-physical', 'baremetal',
+                                           'virtio-forwarder']),
             ],
             support_status=support.SupportStatus(version='2015.1'),
             update_allowed=True,

heat/tests/openstack/neutron/test_neutron_port.py

@@ -1021,6 +1021,11 @@ class UpdatePortTest(common.HeatTestCase):
                                 fixed_ips=None,
                                 addr_pair=None,
                                 vnic_type='baremetal')),
+        ('virtio_forwarder_vnic', dict(secgrp=None,
+                                       value_specs={},
+                                       fixed_ips=None,
+                                       addr_pair=None,
+                                       vnic_type='virtio-forwarder')),
         ('with_all', dict(secgrp=['8a2f582a-e1cd-480f-b85d-b02631c10656'],
                           value_specs={},
                           fixed_ips=[