@Override
    public String getServicePropertyString(String key, String defStr) {
        Object result = serviceProperties.get(key);
        return (result == null) ? defStr : result.toString();
    }
Gets a service property value mapped to the given key
as a String, or null if no such key exists.

@param key a key used for lookup in the service properties
@return the associated String value for the given key, or null
config-model/src/main/java/com/yahoo/vespa/model/AbstractService.java