Yesterday I've been hit by this. every other server was waiting for me
to press enter in order to continue. That process takes forever. The
trick to remove that condition is apparently to run upgrades with cumin
before launching the reboot run.

howto/upgrades.md

@@ -274,6 +274,11 @@ subset of the hosts need a restart. If *all* hosts need a reboot, it's
 likely going to be faster and easier to reboot the entire clusters at
 once, see the [Ganeti reboot procedures](howto/ganeti#rebooting) instead.
 
+NOTE: Reboots will tend to stop for user confirmation whenever packages get
+upgraded just before the reboot. To prevent the process from waiting for your
+manual input, it is suggested that upgrades are run first, using cumin. See
+[how to run upgrades in the section above](#manual-upgrades-with-cumin).
+
 LDAP hosts have information about how they can be rebooted, in the
 `rebootPolicy` field. Here are what the various fields mean: