Require admin context for interfaces on ext network

Currently any user can attach an interface to a neutron
external network, if the neutron plugin supports the port
binding extension.
In this case, nova will create neutron ports using the admin
client, thus bypassing neutron authZ checks for creating ports
on external networks.

This patch adds a check in nova to verify the API request has an
admin context when a request for an interface is made on a
neutron external network.

Conflicts:
	nova/exception.py

Change-Id: I5fb0bdcbf19eb82746ea3b192c1f65899bfb3c0b
Closes-Bug: 1284718
(cherry picked from commit 7d1b4117fda7709307a35e56625cfa7709a6b795)