May agree with deprecating this; not sure. Problem is that the `factory` does provide us with functionality not supported by memberSpec currently. Not sure if that is every definitely required. I know that OpenGamma uses factory in `DynamicRegionsFabric`:
    https://github.com/cloudsoft/brooklyn-opengamma/blob/master/src/main/java/io/cloudsoft/opengamma/cluster/OpenGammaClusterFactory.java
    https://github.com/cloudsoft/brooklyn-opengamma/blob/master/src/main/java/io/cloudsoft/opengamma/app/ElasticOpenGammaApplication.java