Add MachineNetwork to agent-cluster-install generate for dual-stack

For dual-stack installations the agent-cluster-install.yaml must have both an IPv4 and IPv6 subnet in the networkking.MachineNetwork or assisted-service will throw an error. This field is in InstallConfig but it must be added to agent-cluster-install in its Generate().

For IPv4 and IPv6 installs, setting up the MachineNetwork is not needed but it also does not cause problems if its set, so it should be fine to set it all times.