Fix Neutron Authentication for Metadata Service

A recent change to the way Nova creates a Neutron client
51e5f52e4cb60e266ccde71f205c91eb8c97b48b changed the conditions
under which it re-authenticates using the neutron admin credentials
from "if admin" to "if admin or context.is_admin".  However this
meant that any user with admin role in Nova interacted with Neutron
as a different tenant, preventing them from non-admin actions such
as allocating floating IPS.

This was then addressed by 1c1371c78b990447aeaa4377b512f8887e6ff3ce
which only reauthenticated as admin on the explicit use of
"admin=True" parameter to neutonv2.get_client().

However the metadata service does not explicitly create the neutron
client so it has to pass in a context created by
context.get_admin_context() which has is_admin=True but no auth_token.

This change therefore accepts this combination of context values
as a valid reason to re-authenticate as admin, while still allowing
users with the admin role to use their own token.

As a tidy up two tests for getting an admin client are moved from
TestNeutonClient to join the other admin client tests in
TestNeutronClientForAdminScenarios

Change-Id: Ic2a6f383a85f6bb606d8d65feaefa0d23329adb9
Closes-Bug:  1255577