Add --no-parallel to provision

plugins/commands/provision/command.rb

@@ -14,6 +14,11 @@ module VagrantPlugins
                     "Enable only certain provisioners, by type.") do |list|
             options[:provision_types] = list.map { |type| type.to_sym }
           end
+          
+          o.on("--[no-]parallel",
+               "Enable or disable parallelism if provider supports it.") do |parallel|
+            options[:parallel] = parallel
+          end
         end
 
         # Parse the options