Verify args

cloud-init/create_ubuntu20_template.sh

@@ -1,6 +1,15 @@
 #!/bin/sh
 
-_storage=${1}
+_usage() {
+  echo "
+  Usage: $0 <storage>
+  <storage> : should be a valid (active, supporting VM image) PVE storage
+  "
+}
+
+[ -z "$1" ] && _usage && return
+
+_storage=$1
 pvesm status --enabled --target localhost --content images |awk {'print $1'} |tail +2 |grep "$_storage"  > /dev/null 2>&1
 if [ "$?" -ne "0" ]; then
         echo "Storage $_storage does not seems to exists. Use 'pvesm status' to find a usable storage"