Use alternate syntax for disabling ServiceNet

Instead of passing :no_servicenet to rs.network, the following
syntax should be used:

config.vm.provider :rackspace do |rs|
  rs.username = "mitchellh"
  rs.api_key  = "foobarbaz"
  rs.network '443aff42-be57-effb-ad30-c097c1e4503f'
  rs.network :service_net, :attached => false
end