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"
```
It's needed according to passenger docs https://www.phusionpassenger.com/documentation/Users%20guide%20Apache.html#deploying_rails_to_sub_uri (it really does not work without it), not sure about impact on non-suburi deployment.