smart proxy discovery

When user forgets to set `foreman_url` the plugin falls in terrible death. Let's check for this and issue some nice error with 500 HTTP error code.

```
D, [2014-12-15T13:33:27.213703 #5998] DEBUG -- : require_ssl_client_verification: skipping, non-HTTPS request
NoMethodError - undefined method `http_code' for #<URI::InvalidURIError:0x007fc75b90a978>:
        /home/lzap/work/smart_proxy_discovery/lib/smart_proxy_discovery/discovery_api.rb:18:in `rescue in block in <class:Api>'
        /home/lzap/work/smart_proxy_discovery/lib/smart_proxy_discovery/discovery_api.rb:15:in `block in <class:Api>'
```