Add comment into letsencrypt-gencert script clarifying why we stop Apache.

roles/common/files/letsencrypt-gencert

     d="$d,$i.$1";
   fi
 done
+# We are using the "standalone" letsencrypt plugin, which runs its own
+# webserver, so we need to temporarily free up the HTTP(S) ports by stopping
+# our own Apache.
 service apache2 stop
 /root/letsencrypt/letsencrypt-auto certonly -c /etc/letsencrypt/cli.conf --domains $d
 service apache2 start