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
Merge "Refactor yumdownloader conflicts detection"