add digitalocean v2 api support

This commit add v2 api support to digitalocean builder

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Results are now paginated in API v2. This causes an issue when you are trying to reference an image that is not on page 1 of the paginated results. I wonder if there is a way to turn pagination off? If not we have two possible solutions:

A quick and dirty fix would be to add a `per_page` parameter with a very high count so that all results are on the first page. `per_page=200` for example.

Another fix is that when results are paginated, the response includes a `links` attribute that has a reference to the next page. We could iterate through all of the pages and collect the results. The downside to this approach is that each page is an API request.