extract SMW extensions to a YAML file

Having to change the python script anytime we want to tweak the
configuration is a bit cumbersome, instead extract out the list of SMW
extensions to a YAML file which would be easier to change/audit..

We could later on extends the concept to build other bundles such as
i18n or Wikibase.

Change-Id: If7e832201bcefe884ccc5fe44c4572a95757a377