Remove the invalid completion-signal from NovaComputeConfig

Since `NovaComputeDeployment` has `signal_transport: NO_SIGNAL`, this is set to `None` and can override the completion-signal value from other configs on the machine, resulting in the server not being able to send the signal.

If the overcloud's `allNodesConfig` happened to be called `AllNodesConfig`, the compute server would never signal back.