Use ensure-temp-dir in the common.sh script

Instead of having an ensure-temp-dir function in multiple
places, add it to the common.sh script which is sourced by
all the providers.

cluster/common.sh

@@ -308,6 +308,17 @@ function load-or-gen-kube-bearertoken() {
   fi
 }
 
+# Create a temp dir that'll be deleted at the end of this bash session.
+#
+# Vars set:
+#   KUBE_TEMP
+function ensure-temp-dir {
+  if [[ -z ${KUBE_TEMP-} ]]; then
+    KUBE_TEMP=$(mktemp -d -t kubernetes.XXXXXX)
+    trap 'rm -rf "${KUBE_TEMP}"' EXIT
+  fi
+}
+
 # Get the master IP for the current-context in kubeconfig if one exists.
 #
 # Assumed vars: