Adding support to http_proxy and https_proxy env vars to access vCloud REST API

Attempt to fix file_uploader to ise proxy envs. Still one rspec test failing + rubymine complaining on http instanciation. Not clear why.
Adding support to http_proxy and https_proxy env vars to access vCloud REST API

Fixed specs failing

  1) VCloudCloud::FileUploader#upload uploads stream to url
     Failure/Error: nethttphelper.should_receive(:http_proxy).and_return(nil, nil)
       (Double VCloudCloud::NetHttpHelper).http_proxy(any args)
           expected: 2 times with any arguments
           received: 1 time with any arguments
     # ./spec/unit/vcloud/file_uploader_spec.rb:39:in `block (3 levels) in <module:VCloudCloud>'