Simplify job poll

The status of job is:
* submitted: job in queueing system, has no status file.
* submission failed: job not in queueing system, has no status file.
* running: job in queueing system, has status file.
* succeeded/failed: job not in queueing system, has status file.
cylc get-job-status: improve test stability

Improve loadleveler tests stability.
Merge branch 'master' into improve-job-poll
Simplify job poll: remove redundant logic
Merge branch 'improve-job-poll' of https://github.com/matthewrmshin/cylc into improve-job-poll
Merge branch 'master' into improve-job-poll
Remove any old job status file on job submission

And improve stability of get-job-status loadleveler tests.
Merge pull request #808 from matthewrmshin/improve-job-poll

Simplify job poll