Fix typo with $YUM variable

Like ianw suggested, we should fallback to yum if ${YUM} is not
defined.

Change-Id: I9e2c632a3c7225d13e6245e65fa4da29ce7541df
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

nodepool/elements/cache-bindep/install.d/50-cache-pkgs

@@ -55,5 +55,5 @@ if [ -f /usr/bin/apt-get ] ; then
     # --ignore-missing conflicts with set -e, so force it to be ok
     apt-get -y --ignore-missing -d install $PACKAGELIST || true
 else
-    ${YUM} install -y --downloadonly $PACKAGELIST
+    ${YUM:-yum} install -y --downloadonly $PACKAGELIST
 fi