Allows sub-uri deployment

With this patch you can install foreman to sub-uri like this
```
foreman-installer --foreman-foreman-url="https://example.com/foreman" --foreman-proxy-foreman-base-url="https://example.com/foreman"
```
Yes, but it's only way it works. I think it's caused by our hack in https://github.com/theforeman/foreman/blob/develop/config.ru#L6 - we set the prefix manually (so we can run development server under suburi) so having this set to /foreman would end up in double prefixing.