do not create nova volumes endpoints

Since the openstack module no longer supports nova
volumes, this commit sets the cinder flag of
the nova::keystone::auth class which ensures that
nova-volume endpoints will not be configured.

This commit also refactors the unit tests a bit to
use required_params instead of default_params.

Change-Id: I32392bd4ffe2548a02f08fec8467eda2df4bd7c0