public String getServicePropertyString(String key) {
        return getServicePropertyString(key, null);
    }
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