core: Cleaner output for BoxAdd

lib/vagrant/action/builtin/box_add.rb

@@ -281,7 +281,7 @@ module Vagrant
             "vagrant.box_add_with_version",
             name: name,
             version: version,
-            provider: provider))
+            providers: Array(provider).join(", ")))
 
           # Verify the box we're adding doesn't already exist
           if provider && !env[:box_force]

templates/locales/en.yml

@@ -17,7 +17,7 @@ en:
     box_add_choose_provider_again: |-
       Invalid choice. Try again:
     box_add_with_version: |-
-      Adding box '%{name}' (v%{version}) for '%{provider}' provider...
+      Adding box '%{name}' (v%{version}) for provider: %{providers}
     box_added: |-
       Successfully added box '%{name}' for '%{provider}'!
     box_downloading: |-