zuul: make init restart command friendlier

When restarting zuul via /etc/init.d/zuul restart, it just shows up:

 ... waiting

Make it output instead:

 ... waiting for jobs to complete

That is a bit more descriptive.

Change-Id: I607e0823056c2962900790cbd05947e79f2ecf36
Merge "zuul: make init restart command friendlier" into production