Use elevated context as intended

Rectoring introduced in Id8f81419fbbc3616e216e4da0c16898757905933
was not quite complete, and it missed to change the context needed
to create and destroy the metadata network.

Fix bug #1216110

Change-Id: I2161b07f5cd09654f714e237cb4277e8bc60fff1
Merge "Use elevated context as intended" into stable/grizzly
Merge branch 'stable/grizzly' of https://github.com/openstack/quantum into grizzly/stable