builder/digitalocean: add the do builder as a plugin

plugin/builder-digital-ocean/main.go

@@ -0,0 +1,10 @@
+package main
+
+import (
+	"github.com/mitchellh/packer/packer/plugin"
+	"github.com/pearkes/packer/builder/digitalocean"
+)
+
+func main() {
+	plugin.ServeBuilder(new(digitalocean.Builder))
+}