Add a common test case for Port Binding Extension

Add a common test case for Port Binding Extension

This commit merges the test cases for Port Binding Extension in
Linux Bridge plugin and Open vSwitch plugin into a common one.
It makes it easier for each plugin to support the port binding extension.

This commit also adds binding:capabilities attribute to OVS plugin.
Note that binding:capabilities is under discussion and it may be changed
in the future. This change is done just to make OVS plugin support
same as Linux Bridge plugin and pass the common unit test for the extension.

blueprint vif-plugging-improvements

Change-Id: Idc38ea65fe2059e23814d94cd0a0f11a09d1f43e