Port binding registration with NeutronDbPlugin causes Neutron crash

Port binding registration with NeutronDbPlugin causes Neutron crash

fixes bug: 1226169

port binding feature of ML2 plugin registers a callback function with
db_base_plugin_v2.NeutronDbPluginV2, which is invoked during a query of
port DB. This function is registered by name instead by refefence. This
causes wrong context to be passed to the function upon invocation, which
causes the exception. If this query is made during neutron
initilization, Neutron service will fail to start.

This fix changes the registration from function name to function
pointer

Change-Id: I44f7f1a222f80c9ce35f7d49610e52170f76dfd1