Fix Vagrant issue with Salt

Package manager "dnf" does not work correctly with Salt
(cf https://github.com/saltstack/salt/issues/31001)

It causes Salt to consider that some packages (python, git, curl, etc.) are not
installed, which breaks the Vagrant Kubernetes setup.

Updating dnf and dnf-plugins-core to their latest version solves the issue.

Additionally, I've added the "fastestmirror" to dnf, which is useful if a
RPM mirror is broken or very slow. (In my case, dnf used a broken mirror which
froze the Kubernetes setup).

cluster/vagrant/provision-node.sh

@@ -52,6 +52,8 @@ for (( i=0; i<${#NODE_NAMES[@]}; i++)); do
   fi
 done
 
+prepare-package-manager
+
 # Configure network
 if [ "${NETWORK_PROVIDER}" != "kubenet" ]; then
   provision-network-node