don't create scancel commands for users without jobs

[wdpypere]

• make limit of scancel commands configurable
• set limit of amount of user del
• improve logging about the scancel and user del limits
• don't create scancel commands for users that don't exist or don't have any jobs
• fix a silent failure

fixes #127

[wdpypere]

a lot of boilerplate, but this is the main change.

[wdpypere]

so I'm running into a failed test because of:

======================================================================
ERROR: test_slurm_user_accounts (test.slurm_sync.SlurmSyncTestGent)
Test that the commands to create, change and remove users are correctly generated.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/wdpypere/workspace/vsc-administration/test/slurm_sync.py", line 212, in test_slurm_user_accounts
    (job_cancel_commands, commands, remove_user_commands) = slurm_user_accounts(vo_members, active_accounts, slurm_user_info, ["banette"])
  File "lib/vsc/administration/slurm/sync.py", line 376, in slurm_user_accounts
    active_jobs = get_slurm_sacct_active_jobs_for_user(user)
  File "lib/vsc/administration/slurm/sacctmgr.py", line 543, in get_slurm_sacct_active_jobs_for_user
    (exitcode, contents) = asyncloop([SLURM_SACCT, "-L", "-P", "-s", "r", "-u", user])
  File "/home/wdpypere/workspace/vsc-administration/.eggs.py3/vsc_base-3.3.1-py3.6.egg/vsc/utils/run.py", line 151, in run
    return r._run()
  File "/home/wdpypere/workspace/vsc-administration/.eggs.py3/vsc_base-3.3.1-py3.6.egg/vsc/utils/run.py", line 257, in _run
    self._run_pre()
  File "/home/wdpypere/workspace/vsc-administration/.eggs.py3/vsc_base-3.3.1-py3.6.egg/vsc/utils/run.py", line 275, in _run_pre
    self._init_process()
  File "/home/wdpypere/workspace/vsc-administration/.eggs.py3/vsc_base-3.3.1-py3.6.egg/vsc/utils/run.py", line 370, in _init_process
    self._process = self._process_module.Popen(self._shellcmd, **self._popen_named_args)
  File "/usr/lib64/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib64/python3.6/subprocess.py", line 1364, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: '/usr/bin/sacct': '/usr/bin/sacct'

[wdpypere]

I would like mock or patch or whatever get_slurm_sacct_active_jobs_for_user to give a fixed output instead of call sacct but I don't know how to do that.