show help and exit if no argument is provided

Signed-off-by: Jovan Kostovski <chombium@gmail.com>

bugfix for the case where the script was started without an argument.
The help was printed, but the _mainflux_docker was called as well, which
was causing the following error:
./mainflux-docker.sh: line 163: $1: unbound variable

bin/mainflux-docker.sh

@@ -203,6 +203,7 @@ _main() {
   # No arguments provided
   if [[ $# -eq 0 ]] ; then
     _print_help
+    exit 1
   fi
 
   # Avoid complex option parsing when only one program option is expected.