Avoid using sudo to make synced_folder if possible

As of now, making synced_folder needs sudoable nopassword remote user account.
Because of the security impact, it is difficult to use nopassword for us.

So I modified ``synced_folder.rb`` to avoid using sudo to make synced_folder if possible.
By this fix, you can make synced_folder within directory where you have write
permission without nopassword settings.

Could you review and merge this commit?