Only mark environment loaded after load_config

If `load_config` fails, `@loaded` should still be false.

Otherwise, stuff like this fails oddly:

    env = Vagrant::Environment.new(:cwd => '/some/directory')
    begin
      env.cli("up")
    rescue Vagrant::Errors::VagrantError => err
      puts err.message
    ensure
      if env.loaded?
        env.cli("destroy", "-f")
      end
    end