tls-enabled set to true over the keystone-fid-service-provider relation

[Felipe Reyes]

the relation keystone-fid-service-provider informs the subordinate if http or https should be used to talk to keystone (and configure the provider correctly), although when keystone is related to vault (via certificates relation) the tls-enabled key will be set to true even if keystone hasn't been configured to serve by https, this specially a problem when vault is sealed and the certificates may simply never be available during the deployment.

```
    if relation_ids('certificates'):
        tls_enabled = True
```
source: https://opendev.org/openstack/charm-keystone/src/branch/master/hooks/keystone_hooks.py#L835

process_certificates()[0] is capable of detecting if there are certificates available in the relation, but calling it has the side effect that it will write them on disk.

[0] https://opendev.org/openstack/charm-keystone/src/branch/master/hooks/keystone_hooks.py#L872