kernel/v2: boxes can be optional if a provider says so

/cc @fgrehm - For the DOcker provider, we can now make boxes optional. :)
No more dummy boxes!

plugins/kernel_v2/config/vm.rb

@@ -469,7 +469,11 @@ module VagrantPlugins
 
       def validate(machine)
         errors = _detected_errors
-        errors << I18n.t("vagrant.config.vm.box_missing") if !box
+
+        if !box && !machine.provider_options[:box_optional]
+          errors << I18n.t("vagrant.config.vm.box_missing")
+        end
+
         errors << I18n.t("vagrant.config.vm.hostname_invalid_characters") if \
           @hostname && @hostname !~ /^[a-z0-9][-.a-z0-9]+$/i
 

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -28,6 +28,7 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
     env.stub(root_path: nil)
     machine.stub(env: env)
     machine.stub(provider_config: nil)
+    machine.stub(provider_options: {})
 
     subject.box = "foo"
   end
@@ -44,6 +45,21 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
     end
   end
 
+  describe "#box" do
+    it "is required" do
+      subject.box = nil
+      subject.finalize!
+      assert_invalid
+    end
+
+    it "is not required if the provider says so" do
+      machine.provider_options[:box_optional] = true
+      subject.box = nil
+      subject.finalize!
+      assert_valid
+    end
+  end
+
   context "#box_check_update" do
     it "defaults to true" do
       subject.finalize!