kernel/v2: allow provisioner plugins to register without config

plugins/kernel_v2/config/vm_provisioner.rb

@@ -49,6 +49,7 @@ module VagrantPlugins
         if !@config_class
           @logger.info(
             "Provisioner config for '#{@name}' not found. Ignoring config.")
+          @config_class = Vagrant::Config::V2::DummyConfig
         end
       end
 

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

@@ -3,6 +3,8 @@ require File.expand_path("../../../../base", __FILE__)
 require Vagrant.source_root.join("plugins/kernel_v2/config/vm")
 
 describe VagrantPlugins::Kernel_V2::VMConfig do
+  include_context "unit"
+
   subject { described_class.new }
 
   let(:machine) { double("machine") }
@@ -210,6 +212,24 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
       expect(r[0]).to be_invalid
     end
 
+    it "allows provisioners that don't define any config" do
+      register_plugin("2") do |p|
+        p.name "foo"
+        # This plugin registers a dummy provisioner
+        # without registering a provisioner config
+        p.provisioner(:foo) do
+          Class.new Vagrant::plugin("2", :provisioner)
+        end
+      end
+
+      subject.provision("foo") do |c|
+        c.bar = "baz"
+      end
+
+      # This should succeed without errors
+      expect{ subject.finalize! }.to_not raise_error
+    end
+
     describe "merging" do
       it "copies the configs" do
         subject.provision("shell", inline: "foo")