keystone-ssl-ca config option should override keystone-credentials:ca_cert relation data

[Adam Dyess]

keystone-ssl-ca config option appropriately defaults to empty
https://github.com/charmed-kubernetes/charm-kubernetes-master/blob/master/config.yaml#L268-L273

If this option is not empty, and keystone is related via the keystone-credentials relation, the charm should attempt to use the ca_cert option from the relation's endpoint.

1) use the configured value
2) use the relation value from keystone-credentials endpoint if available
See definition of relation: https://github.com/openstack/charm-interface-keystone-credentials/blob/53e93b8820899f2251d207ed5d5c3b212ceb64de/requires.py#L20-L35 >
3) Remain empty and assume that keystone is rooted in a public cert