Add a YUM variable that defaults to dnf for Fedora 22 and greater. At
this stage the yum element can do double-duty with dnf -- it's mostly
the same. If we find it starts getting too unwieldy we can separate
this later.
Modify the install-packages for yum to use this variable when set, but
default back to "yum" to retain the status-quo.
Change-Id: Ibff71465b392d9f66b6f93955ff9223575d6165c
