Ensure provider specific config gets transformed into common form

plugins/kernel_v2/config/disk.rb

@@ -101,7 +101,7 @@ module VagrantPlugins
 
         current = @provider_config.merge(current) if !@provider_config.empty?
         if current
-          @provider_config = current[:provider_config]
+          @provider_config = current
         else
           @provider_config = {}
         end

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

@@ -115,4 +115,12 @@ describe VagrantPlugins::Kernel_V2::VagrantConfigDisk do
       assert_invalid
     end
   end
+
+  describe "#add_provider_config" do
+    it "normalizes provider config" do
+      test_provider_config = {provider__something: "special" }
+      subject.add_provider_config(test_provider_config)
+      expect(subject.provider_config).to eq( { provider: {something: "special" }} )
+    end
+  end
 end

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

@@ -669,6 +669,16 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
       expect(merged_disks[0].name).to eq("foo")
       expect(merged_disks[1].name).to eq("bar")
     end
+
+    it "adds provider config with `__` config form" do
+      subject.disk(:disk, size: 1000, primary: false, name: "storage",  provider__something: "special")
+      expect(subject.disks[0].provider_config).to eq({:provider=>{:something=>"special"}})
+    end
+
+    it "adds provider config with Hash config form" do
+      subject.disk(:disk, size: 1000, primary: false, name: "storage",  provider: {something: "special"})
+      expect(subject.disks[0].provider_config).to eq({:provider=>{:something=>"special"}})
+    end
   end
 
   describe "#synced_folder(s)" do