Do not trigger agent notification if bindings do not change

The method _process_port_bindings for the ml2 plugin should not
return True when the host binding does not change, otherwise an
unnecessary notification will be sent to the agent

Closes-Bug: #1255680
Related-Bug: #1253896
Partially Implements: neutron-tempest-parallel

Change-Id: I8a40090af347ca430ff6c8e2211fa34bb2bd0f8c
Merge "Do not trigger agent notification if bindings do not change"