KeyError: 'tm_events_per_lumi' with old crabserver

@matz-e @mmascher

With old crabserver it is not working
```
Traceback (most recent call last):
  File "/data/srv/TaskManager/3.3.12.pre2/slc6_amd64_gcc481/cms/crabtaskworker/3.3.12.pre2/lib/python2.6/site-packages/TaskWorker/Actions/Handler.py", line 58, in actionWork
    output = work.execute(nextinput, task=self._task)
  File "/data/srv/TaskManager/3.3.12.pre2/slc6_amd64_gcc481/cms/crabtaskworker/3.3.12.pre2/lib/python2.6/site-packages/TaskWorker/Actions/DagmanCreator.py", line 742, in execute
    return self.executeInternal(*args, **kw)
  File "/data/srv/TaskManager/3.3.12.pre2/slc6_amd64_gcc481/cms/crabtaskworker/3.3.12.pre2/lib/python2.6/site-packages/TaskWorker/Actions/DagmanCreator.py", line 734, in executeInternal
    info = self.createSubdag(*args, **kw)
  File "/data/srv/TaskManager/3.3.12.pre2/slc6_amd64_gcc481/cms/crabtaskworker/3.3.12.pre2/lib/python2.6/site-packages/TaskWorker/Actions/DagmanCreator.py", line 603, in createSubdag
    jobgroupspecs, startjobid = self.makeSpecs(kwargs['task'], sitead, siteinfo, jobgroup, block, availablesites, outfiles, startjobid)
  File "/data/srv/TaskManager/3.3.12.pre2/slc6_amd64_gcc481/cms/crabtaskworker/3.3.12.pre2/lib/python2.6/site-packages/TaskWorker/Actions/DagmanCreator.py", line 500, in makeSpecs
    'eventsPerLumi' : task['tm_events_per_lumi'],
KeyError: 'tm_events_per_lumi'
```