openstack_config contains a method 'self.instances' that is required for
purging unmanaged resources that references a static namevar method that
is not defined in openstack_config and may not be provided by base
classes. The 'namevar' method doesn't require any information from the
derived class so a default implementation in the base provider type
suffices.
Closes-Bug: #1612009
Change-Id: If27152863b9df5a28bf45e23ca989019666a958c