New digitalocean_volume_attachment resource for proper updates of droplet volumes

[TFaga]

This PR adds an additional virtual resource named digitalocean_volume_attachment, which represents a volume attachment to a droplet. Replaces the current way of attaching volumes by passing in their ids to the droplet definition.

Using the attachment resource allows terraform to correctly traverse the dependency graph of our resources and enables the provider to gracefully update any attached volumes. It does so by first detaching the volumes from the droplet, deleting old ones, creating new ones and finally reattaching the new volumes to the initial droplet.

Backward compatibility is preserved. You can continue using the current way of attaching volumes to droplets without issue. Just keep in mind that the methods are mutually exclusive. You must not use both of them at the same time. The documentation is also updated accordingly. Fixes #87

Example of the attachment resource:

resource "digitalocean_volume" "foo" {
  region = "lon1"
  name = "foo"
  size = 5
}

resource "digitalocean_droplet" "foo" {
  name = "foo"
  region = "lon1"
  size = "s-1vcpu-1gb"
  image = "ubuntu-18-04-x64"
}

resource "digitalocean_volume_attachment" "foo" {
  droplet_id = "${digitalocean_droplet.foo.id}"
  volume_id = "${digitalocean_volume.foo.id}"
}

[andrewsomething]

Great stuff!