Added support for Digital Ocean droplet tags

[mblarsen]

As discussed in #51

[adammck]

Wonderful, thank you!

[derfabianpeter]

Is it possible that this doesn't work anymore on TF 0.12.x and terraform-inventory 0.9? I can successfully tag my Droplets, but ansible only used the resource-identifier to group, not the droplet tags:

resource "digitalocean_droplet" "manager" {
  count = var.manager_count
  ssh_keys           = var.ssh_key_ids
  image              = "ubuntu-18-04-x64"
  region             = "fra1"
  size               = "s-6vcpu-16gb"
  private_networking = true
  backups            = false
  monitoring         = true
  ipv6               = true
  name               = "manager-${count.index+1}"
  tags               = ["manager", "swarm", "docker", "cio"]
}

Results in:

[all]
manager-1
manager-2
manager-4
manager-3

[all:vars]

[manager]
manager-1
manager-2
manager-3
manager-4

[manager.0]
manager-1

[manager.1]
manager-2

[manager.2]
manager-3

[manager.3]
manager-4

[type_digitalocean_droplet]
manager-1
manager-2
manager-4
manager-3

[akuznecov]

@derfabianpeter just build terraform-inventory from sources and it will work rather than one from Releases page.