Add downloadonly flag to fedora

devstack-gate needs to be able to make an image that has pre-downloaded
but not installed a sequence of packages. To support that, add a -d flag
to install-packages to allow downloading but not installing. -d passes
through on a dpkg install, and with this patch should be transformed
to --downloadonly which is the yum version.

Change-Id: Ia4e61fa4304df86afbee7dfc8067ea1bdf528f3e

elements/fedora/bin/install-packages

@@ -20,9 +20,12 @@ set -e
 if [ "$1" = "-u" ] ; then
   yum -y update
   exit 0
+elif [ "$1" = "-d" ] ; then
+  EXTRA_ARGS="--downloadonly"
+  shift
 fi
 
-yum -y install $(map-packages "$@")
+yum -y install $EXTRA_ARGS $(map-packages "$@")
 for pkg in "$@"; do
   if [ "$pkg" = "python-pip" ] ; then
     alternatives --install /usr/bin/pip pip /usr/bin/pip-python 10