Users with admin role in Nova should not re-auth with Neutron

A recent change to the way Nova creates a Neutron clients
https://review.openstack.org/#/c/52954/4 changed the conditions
under which it re-authenticates using the neutron admin credentials
from "if admin" to "if admin or context.is_admin".

This means that any user with admin role in Nova now interacts
with Neutron as a different tenant.  Not only does this cause an
unnecessary re-authentication (The user may/should also have an admin
role in Neutron) it means that they can no longer allocate and assign
a floating IP to their instance via Nova (as the floating ip will now
always be allocated in the context of neutron_admin_tenant).

This change removes the context.is_admin check to revert to the
previous behaviour where the used of admin creds is determined just
on the explicit use of "admin=True" parameter to get_client().

Change-Id: Ib1720420c778960bc90c5b7d703de936ebb7d6b5
Closes-Bug: 1250763
Merge "Users with admin role in Nova should not re-auth with Neutron"