[ops-guide] Update reference to proprietary Cinder drivers
[ops-guide] Update reference to proprietary Cinder drivers

Remove very old outdated list of proprietary options to just link to
current generated list of all drivers.

Change-Id: I8dab0c32aedee82466f508da39b0d49e2606d046

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals