Support noauth authentication patterns for neutron-client

As per https://github.com/openstack/python-neutronclient/blob/master/neutronclient/client.py#L216, alternate authentication methods are unsupported.

This is a predecessor of another blueprint which will implement truly pluggable auth (see: https://blueprints.launchpad.net/python-neutronclient/+spec/pluggable-neutronclient-auth ).

In this blueprint, I'm proposing a simple branching logic to support a noauth authentication pattern.