This fixes a bug in the condor plugin

I am afraid there is another problem in the condor plugin, today tier0 jobs got massively killed with no error messages... when I checked, there was a problem in the logic at: 
https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/BossAir/Plugins/CondorPlugin.py#L724

The problem is that when jobAd.get('DESIRED_Sites') returns 'T2_CH_CERN_T0', the check in that line for  'T2_CH_CERN' is True because it checks for the string itself. Then when T2_CH_CERN is set to down is actually killing all the jobs for T2_CH_CERN_T0 !!! 

This patch should avoid this problem in the future.