neutron: Support ml2 port bindings

In order to use the ml2 core plugin, a port bindings extension for the
port should be supported. By providing the Port.hostId, the ml2 plugin
detects the agent configuration on which the port is allocated and
configures the port entity properly.

Signed-off-by: Moti Asayag <masayag@redhat.com>
Merge pull request #155 from masayag/master

neutron: Support ml2 port bindings