Switch gnt-debug submit-job to JobExecutor

Currently gnt-debug submits jobs individually, but in 2.1 JobExecutor
uses the optimized SubmitManyJobs luxi call and as such should be used
whenever multiple jobs need to be submitted.

This patch converts gnt-debug submit-job to use it and also removes an
extra empty line in the JobExecutor class.

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