Full review (do not merge)

It appears that I can't just leave http_rackup_path because then smart-proxy fails with
```
/usr/share/foreman-proxy/lib/smart_proxy.rb:78:in `instance_eval': no implicit conversion of nil into String (TypeError)
from /usr/share/foreman-proxy/lib/smart_proxy.rb:78:in `block (2 levels) in http_app'
from /usr/share/foreman-proxy/lib/smart_proxy.rb:78:in `each'
from /usr/share/foreman-proxy/lib/smart_proxy.rb:78:in `block in http_app'
from /usr/share/gems/gems/rack-1.4.5/lib/rack/builder.rb:51:in `instance_eval'
from /usr/share/gems/gems/rack-1.4.5/lib/rack/builder.rb:51:in `initialize'
from /usr/share/foreman-proxy/lib/smart_proxy.rb:77:in `new'
from /usr/share/foreman-proxy/lib/smart_proxy.rb:77:in `http_app'
from /usr/share/foreman-proxy/lib/smart_proxy.rb:123:in `launch'
from /usr/share/foreman-proxy/bin/smart-proxy:6:in `<main>'
```

Should smart-proxy be changed to allow this?