Give layout objects a unique ID

This is a debug aid -- when investigating leaked layout objects,
it would be nice to tie them back to when they were created.

Change-Id: I2b5b3cb33e6891c0cbca15ea82022bfa91ba9048
diff --git a/zuul/model.py b/zuul/model.py
index e68e46b..464ee16 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -2329,6 +2329,7 @@
     """Holds all of the Pipelines."""
 
     def __init__(self, tenant):
+        self.uuid = uuid4().hex
         self.tenant = tenant
         self.project_configs = {}
         self.project_templates = {}