install-labmanager: myuseradd: fix the error when a gid was specified.

install-labmanager.sh

@@ -49,7 +49,7 @@ myuseradd() {
   fi
   R_GID=""
   if [ "z$3" != "z" ]; then
-    R_GID="$3"
+    R_GID="-g $3"
   fi
   useradd -b /local -m ${R_UID} -U ${R_GID} -r ${R_USER}
   if [ $? -ne 0 ]; then