Switch to dnf when it exists

This has all been around for a *long* time, like when dnf was a weird
new thing.  Now it's the opposite and yum is a weird old thing :)
Choose it by default for platforms with it (Fedora, for now).

Change-Id: Id2bd7d145354b996de31944929fd0267ec24a08e

Author: ianw

stackrc

@@ -819,11 +819,14 @@ SERVICE_GRACEFUL_SHUTDOWN_TIMEOUT=${SERVICE_GRACEFUL_SHUTDOWN_TIMEOUT:-5}
 # Service graceful shutdown timeout
 WORKER_TIMEOUT=${WORKER_TIMEOUT:-90}
 
-# Support alternative yum -- in future Fedora 'dnf' will become the
-# only supported installer, but for now 'yum' and 'dnf' are both
-# available in parallel with compatible CLIs.  Allow manual switching
-# till we get to the point we need to handle this automatically
-YUM=${YUM:-yum}
+# Choose DNF on RedHat/Fedora platforms with it, or otherwise default
+# to YUM.  Can remove this when only dnf is supported (i.e. centos7
+# disappears)
+if [[ -e /usr/bin/dnf ]]; then
+    YUM=${YUM:-dnf}
+else
+    YUM=${YUM:-yum}
+fi
 
 # Common Configuration
 # --------------------