add digitalocean v2 api support

This commit add v2 api support to digitalocean builder

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
I think this should be of type `float64`. I haven't confirmed with API v1 but in API v2 the transfer rates are expressed as `1.0`, `2.0`, `7.0`, etc.