JobCoordinator: Retry the task submission for a limited number of failures.

There is a lot of contension on the disco server so the task submission may
timeout.  We should not use large timeouts for submitting the tasks because
meanwhile the job coordinator process is blocked.
A benefit of smaller timeout is that job_coordinator can still process the
messages from other processes.
Tests: Add a sleep to the test_kill to make it more reliable.

Killing a job casts a couple of messages in the backend and it is not guaranteed
that the job_coordinator is dead when the http request is responded.  Therefore,
test_kill which wants to make sure the process is dead just after killing the
job, is not reliable.
Merge pull request #469 from pooya/task_submit

Task submit