[domains] make sure we match the specific string in VHOST

in response to https://github.com/progrium/dokku/commit/4e0670f1ac2fd2772c6b3e7298e779c5c3fb064e#commitcomment-9056929

Make sure we can allow VHOSTS addition in the order of
```
web-app.example.com
app.example.com
```