Fix jstart to start only one job instance.

https://wikitech.wikimedia.org/wiki/Nova_Resource:Tools/Help#Continuous_jobs_.28such_as_bots.29
promises that "jstart script" "is equivalent to jsub -once -continuous
script".  This fixes bug #48816.

Bug: 48816
Change-Id: Ie3a70c7418d5ea2897a4b331c50f6adf0fb7f816