Fix parameters for QueryJob in JobExecutor

When gnt-job submit is used with the --each option, it sends as
parameters for QueryJobs a list of lists, each containing a job ID.
While this works now, it is not the intended format for QueryJobs.

This patch fixes this behavior, by making it send a list of job IDs.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>