distribution: collect as a dry run until every kept bundle is held by a manifest

The stock collector keeps only what a manifest names, and the mesh's bundles
and archives are bare blobs no manifest names: its first run, tonight at
03:30, would delete every one, the current ones included. It now reports
what it would delete and deletes nothing, until the controller holds each
kept archive with a manifest. hq issue 253.

modules/distribution/module.json

@@ -74,6 +74,7 @@
       ],
       "args": [
         "garbage-collect",
+        "--dry-run",
         "/etc/docker/registry/config.yml"
       ],
       "schedule": "30 3 * * *",