Resolves a bug encountered when setting the Nova scheduler to
be aware of Neutron routed provider network segments, by using
'query_placement_for_routed_network_aggregates'.
Non-admin users attempting to access the 'segment_id' attribute
of a subnet caused a traceback, resulting in instance creation
failure.
This patch ensures the Neutron client is initialised with an
administrative context no matter what the requesting user's
permissions are.
Change-Id: Ic0f25e4d2395560fc2b68f3b469e266ac59abaa2
Closes-Bug: #1970383
ee32934f34