NSX: Correct allowed_address_pair return value on create_port

This patch fixes the NSX plugin to return [] rather than None for a
port does not have any allowed_address_pairs when a create_port api
call is made. Currently the NSX plugin is failing every patch because
now tempest is checking for this value to be [].

Change-Id: Ic82c564ff62000c367c544f44a76694be9375790
Closes-bug: 1366921
Merge "NSX: Correct allowed_address_pair return value on create_port"