[openstack] add remove header if setting directory back to private

This is needed if you want to put you container back to private. Since sending empty header wont do it. 

To nitpick a bit, this would be cleaner as:

```ruby
if options[:public]
  headers['X-Container-Read'] = '.r:*'
else
  headers['X-Remove-Container-Read'] = 'x'
end
```