Add a warning if dotfile_name is used in V1

plugins/kernel_v1/config/vagrant.rb

@@ -11,10 +11,20 @@ module VagrantPlugins
         @host         = UNSET_VALUE
       end
 
-      def upgrade(new)
-        new.vagrant.host = @host if @host != UNSET_VALUE
+      def finalize!
+        @dotfile_name = nil if @dotfile_name == UNSET_VALUE
+        @host = nil if @host == UNSET_VALUE
+      end
 
-        # TODO: Warn that "dotfile_name" is gone in V2
+      def upgrade(new)
+        new.vagrant.host = @host if @host.nil?
+
+        warnings = []
+        if @dotfile_name
+          warnings << "`config.vm.dotfile_name` has no effect anymore."
+        end
+
+        [warnings, []]
       end
     end
   end