add ability to specify api url

[vtolstov]

please add ability to specify api url, because some providers have DO like api to creating vps, and previously packer use handmade lib to support DO and have this ability, but in last release it switches to this library and i miss this feature.

[bryanl]

You can specify the API URL by updated the BaseURL field of godo.Client. https://github.com/digitalocean/godo/blob/master/godo.go#L36

// set up oauthClient like in the README
client := godo.NewClient(oauthClient)
u, err := url.Parse("http://example.com")
client.BaseURL = u

[vtolstov]

Thanks!