Modify the way to get 'network_id' of the port

This patch change to get 'network_id' from port info, instead of
from subnet info.

Implements blueprint implement-ec2eip-updatable

Change-Id: I51ed7e71af4c7223df9e399e189389fd655a9746
Merge "Modify the way to get 'network_id' of the port"