zfs: the scheduled run names its minutes, so its timer starts (issue 1979)

The node-engine writes a cron step (*/15) as a calendar systemd refuses, so
zfs-mesh-scrub.timer never started and the pool had no scrub schedule. A list
of minutes is written as *-*-* *:0,15,30,45:00, which systemd takes.

Builds: design 42 zfs-owns-zfs §18. Tests: 1979/AC1.

modules/zfs/module.json

@@ -195,7 +195,7 @@
         "./zfs-tools",
         "scrub-run"
       ],
-      "schedule": "*/15 * * * *"
+      "schedule": "0,15,30,45 * * * *"
     },
     {
       "id": "distribution-weekly",