pipeline: introduce generic pipeline id

This is a step towards generic pipelines, i.e. replacing assemblers
with pipelines, thus creating an acyclic graph of pipelines. There
the pipeline id will be what is now the tree_id. For now though the
generic id is either the output_id or the tree_id.

osbuild/pipeline.py

@@ -147,6 +147,10 @@ class Pipeline:
         self.stages = []
         self.assembler = None
 
+    @property
+    def id(self):
+        return self.output_id or self.tree_id
+
     @property
     def tree_id(self):
         return self.stages[-1].id if self.stages else None