Add YUM variable to for Fedora >= 22

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

elements/fedora/environment.d/11-yum-dnf.bash

@@ -0,0 +1,8 @@
+# since f22, dnf is the yum replacement.  Mostly it drops in
+# unmodified, so while we transision KISS and use this to choose
+
+if [ $DIB_RELEASE -ge 22 ]; then
+    export YUM=dnf
+else
+    export YUM=yum
+fi

elements/yum/bin/install-packages

@@ -24,6 +24,9 @@ EXTRA_ARGS=
 ACTION=install
 MAP_ELEMENT=
 
+# allow override for dnf, as shipped by default with >=F22
+YUM=${YUM:-yum}
+
 SCRIPTNAME=$(basename $0)
 function show_options () {
     echo "Usage: $SCRIPTNAME [package ...]"
@@ -42,7 +45,7 @@ eval set -- "$TEMP"
 
 while true ; do
     case "$1" in
-        -u) yum -y update; exit 0;;
+        -u) ${YUM} -y update; exit 0;;
         -d) EXTRA_ARGS="--downloadonly"; shift;;
         -e) ACTION="erase"; shift;;
         -m) MAP_ELEMENT=$2; shift 2;;
@@ -52,6 +55,7 @@ while true ; do
     esac
 done
 
+
 # Packages that aren't available in the distro but requested for installation
 # can be ignored by adding them to the exclude list
 BLACKLIST=$(cat /tmp/yum-blacklist 2>/dev/null || echo "")
@@ -87,7 +91,7 @@ if [ -n "$WHITELIST" ]; then
         echo "Not running install-packages $ACTION with empty packages list"
     else
         echo "Running install-packages ${ACTION}. Package list: $PKGS"
-        yum -y $ACTION $EXTRA_ARGS $PKGS
+        ${YUM} -v -y $ACTION $EXTRA_ARGS $PKGS
         for pkg in "$@"; do
             if [ "$pkg" = "python-pip" ] ; then
                 alternatives --install /usr/bin/pip pip /usr/bin/pip-python 10