Refactor yumdownloader conflicts detection
Refactor yumdownloader conflicts detection

On the modern OSes (like Ubuntu-13.10 or CentOS-6) the following
commands return nothing:

  yumdownloader -q --resolve ... > yumdownloader.log
  grep "has depsolving problems" yumdownloader.log

Which breaks our conflicts detections mechanism during ISO build.

Change-Id: I67e4eefaba4340f84730515e59a806adba184af1

stackforge/fuel-main,eayunstack/fuel-main,dancn/fuel-main-dev,zhaochao/fuel-main,Fiware/ops.Fuel-main-dev,stackforge/fuel-main,AnselZhangGit/fuel-main,dancn/fuel-main-dev,SergK/fuel-main,teselkin/fuel-main,stackforge/fuel-main,ddepaoli3/fuel-main-dev,teselkin/fuel-main,ddepaoli3/fuel-main-dev,teselkin/fuel-main,SmartInfrastructures/fuel-main-dev,SergK/fuel-main,Fiware/ops.Fuel-main-dev,zhaochao/fuel-main,AnselZhangGit/fuel-main,SmartInfrastructures/fuel-main-dev,zhaochao/fuel-main,teselkin/fuel-main,Fiware/ops.Fuel-main-dev,AnselZhangGit/fuel-main,eayunstack/fuel-main,ddepaoli3/fuel-main-dev,SmartInfrastructures/fuel-main-dev,huntxu/fuel-main,SergK/fuel-main,ddepaoli3/fuel-main-dev,zhaochao/fuel-main,SmartInfrastructures/fuel-main-dev,AnselZhangGit/fuel-main,zhaochao/fuel-main,dancn/fuel-main-dev,Fiware/ops.Fuel-main-dev,huntxu/fuel-main,dancn/fuel-main-dev,huntxu/fuel-main,eayunstack/fuel-main