Modify cli.JobExecutor to use SubmitManyJobs

This patch changes the generic "multiple job executor" to use the many
jobs submit model, which automatically makes all its users use the new
model.

This makes, for example, startup/shutdown of a full cluster much more
logical (all the submitted job IDs are visible fast, and then waiting
for them proceeds normally).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>