keystone identity-credentials relation is missing ca_cert field

https://github.com/openstack/charm-interface-keystone-credentials/blob/53e93b8820899f2251d207ed5d5c3b212ceb64de/requires.py#L20-L35 defines the keystone-credentials interface, however the keystone charm doesn't provide `ca_cert` on this relation.

https://github.com/openstack/charm-keystone/blob/4df4ed6da409c967ef0ca4de6a5c2b19960a1a74/hooks/keystone_utils.py#L2010

but instead opts for __null__

If the relation reported this, the cert for connecting to a TLS encrypted Keystone would be obvious rather than something requiring user configuration -- as is the case for kubernetes-master::keystone-ssl-ca

https://github.com/charmed-kubernetes/charm-kubernetes-master/blob/master/config.yaml#L268-L273