Add an extension to show the network id of a virtual interface

This extension adds the OS-EXT-VIF-NET:net_id param
so that users can determine vif plunged into which network now.

DocImpact

Change-Id: I56bef5c45f7f545aa5e9b13760dff1c802da0f8f
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Add an extension to show the network id of a virtual interface"