Fix argument name for passing CA certificates to Keystone client
Fix argument name for passing CA certificates to Keystone client

`certificates` -> `cacerts`

See
https://docs.openstack.org/python-keystoneclient/queens/api/keystoneclient.v2_0.html#module-keystoneclient.v2_0.client
for a description of the `cacert` parameter for initializing a
Keystone v2 client with a CA certificate bundle.

eglute/copystack