baserock-source volume doesn't seem necessary for the default build

The supplied default.benchmark uses a volume called baserock-source and explains how to build it. However, you can remove the mounts section of the test, or have a definition of the baserock-source volume which contains no preparation script. This looks to me like the default benchmark doesn't need any of the things in the volume and is fetching them itself, which is confusing.

The current version also appears to fetch and install the bst-external repository in both the volume creation and the test itself, which is strange.