CA: store credentials in file

Right now supports storing user credentials in the config file directly:

"authentication": {
            "type": "basic",
            "realm": "kea-control-agent",
            "clients": [
            {
                "user": "admin",
                "password": "1234"
            } ]
        }

This is considered insecure as it shows on all config-get results and is being shown in Stork for example. An alternative would be to add credentials file, a file that would contain the same content as clients: [ ... ].