Allow user to choose NFS protocol version, defaulting to 3

NFSv4 has siginificant added complexity in the form of ID-mapping which
can cause problems with applications without extra setup. Best to
force version 3 unless the user requests it.

Conflicts:

	lib/vagrant/systems/linux.rb

config/default.rb

@@ -26,6 +26,7 @@ Vagrant::Config.run do |config|
 
   config.nfs.map_uid = :auto
   config.nfs.map_gid = :auto
+  config.nfs.version = 3
 
   config.package.name = 'package.box'
 end

lib/vagrant/config/nfs.rb

@@ -3,6 +3,7 @@ module Vagrant
     class NFSConfig < Base
       attr_accessor :map_uid
       attr_accessor :map_gid
+      attr_accessor :version
     end
   end
 end

lib/vagrant/guest/linux.rb

@@ -60,7 +60,7 @@ module Vagrant
 
           # Do the actual creating and mounting
           @vm.channel.sudo("mkdir -p #{real_guestpath}")
-          @vm.channel.sudo("mount #{ip}:'#{opts[:hostpath]}' #{real_guestpath}",
+          @vm.channel.sudo("mount -o vers=#{opts[:version]} #{ip}:'#{opts[:hostpath]}' #{real_guestpath}",
                           :error_class => LinuxError,
                           :error_key => :mount_nfs_fail)
         end