refactor port binding codes

Bug #1195047

This patch:
1. removes the iteration in get_ports to call extend_dict_binding
2. uses a unified way for plugins to do this kind of stuff

ml2 will enhance the binding so that it can have different binding
for port according to port's host.
mlnx and bigswitch are also exceptions due to the dynamic binding info.

Change-Id: I5a77eb7395e14482a856e033f536f72a1bf82e06
Merge "refactor port binding codes"