Use DNF instead of YUM as default OS installer for f22+
Use DNF instead of YUM as default OS installer for f22+

ros-infrastructure/rosdep,spaghetti-/rosdep,wkentaro/rosdep,allenh1/rosdep,spaghetti-/rosdep,alessandro-aglietti/rosdep,alessandro-aglietti/rosdep,allenh1/rosdep,wkentaro/rosdep,ros-infrastructure/rosdep