added upgrade feature for 5.5.2 edges deployed with 6.0.x nsx

added upgrade feature for 5.5.2 edges deployed with 6.0.x nsx
this method is somewhat inconsistent with the rest of the provider, which assumes that
* an invocation of 'exists?' that does not return false will set @instance
* @instance has_key?('id')

So for consistency, valuable to readers of the future, I would suggest
```ruby
    def flush
      return unless exists?
      post("api/4.0/edges/#{@instance['id']}?action=upgrade") if resource[:upgrade]
    end
```