osbuild.py/pipeline: add the concept of a content store

Whenever an assembler is not specified, the output tree is instead
saved to the content store, in a directory named after the pipeline
id.

This should render the io.weldr.tree assembler redundant.

In order to build the samples as before, specify the content store
as the input directory to build any pipeline that uses the
io.weldr.untree stage.

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base.json

@@ -20,12 +20,5 @@
         ]
       }
     }
-  ],
-  "assembler":
-    {
-      "name": "io.weldr.tree",
-      "options": {
-        "tree": "base"
-      }
-    }
+  ]
 }