Add missing cinder_client config

`tox -egenconfig` didn't generate cinder_client options
to config file.
This patch fixes it.

Change-Id: I1f580e447ef700761e169971e398e21ad6dd8216
Closes-Bug: #1554399

magnum/opts.py

@@ -51,6 +51,7 @@ def list_opts():
         ('heat_client', magnum.common.clients.heat_client_opts),
         ('glance_client', magnum.common.clients.glance_client_opts),
         ('barbican_client', magnum.common.clients.barbican_client_opts),
+        ('cinder_client', magnum.common.clients.cinder_client_opts),
         ('nova_client', magnum.common.clients.nova_client_opts),
         ('neutron_client', magnum.common.clients.neutron_client_opts),
         ('x509', magnum.common.x509.config.x509_opts),