labs: exit for invalid syntax

Make osbash exit with an explanation if no option for building VMs (-b),
creating Windows batch scripts (-w), or exporting node VMs (-e) is given
(there is nothing to do).

Change-Id: I2376a792fcd6303c64a21f4378acb15bd99a476f

labs/osbash.sh

@@ -154,6 +154,12 @@ if [ -n "${EXPORT_VM_DIR:-}" ]; then
     exit
 fi
 
+if [ -z "${OSBASH:-}" -a -z "${WBATCH:-}" ]; then
+    echo
+    echo "No -b, -w, or -e option given. Exiting."
+    exit
+fi
+
 echo >&2 "$(date) osbash starting"
 
 clean_dir "$LOG_DIR"