Refactor: Rename Cargo into Pod

[leon3s]

People seems to find confusing the Cargo terminology, we may replace it into Pod

Let me know what you think about it.

[unixfox]

Just like nomad has his own naming for defining what a workload is called, it's great that nanocl has its own name too. The naming doesn't have to resemble to Kubernetes, and it's great if you do some search about cargo you won't end up on some Kubernetes tutorial. Whereas with the "Pod" naming, you could very well have mixed results for nanocl and Kubernetes.