Setting default FlexVolume driver directory on COS images.

cluster/common.sh

@@ -745,6 +745,12 @@ EOF
 FEATURE_GATES: $(yaml-quote ${FEATURE_GATES})
 EOF
   fi
+  if [[ "${master}" == "true" && "${MASTER_OS_DISTRIBUTION}" == "gci" ]] ||
+     [[ "${master}" == "false" && "${NODE_OS_DISTRIBUTION}" == "gci" ]]; then
+    cat >>$file <<EOF
+VOLUME_PLUGIN_DIR: $(yaml-quote ${VOLUME_PLUGIN_DIR:-/home/kubernetes/flexvolume})
+EOF
+  fi
 
   if [ -n "${PROVIDER_VARS:-}" ]; then
     local var_name