Ensure that YAML marshalling maintains field ordering

[elezar]

When outputing the following spec as YAML:

cdi.Spec{
	Version: "v0.3.0",
	Kind:    "example.com/class",
	ContainerEdits: cdi.ContainerEdits{
	DeviceNodes: []*cdi.DeviceNode{
		{
			Path: "/dev/foo",
		},
	},
}

We get the following output:

---
cdiVersion: v0.3.0
containerEdits:
  deviceNodes:
  - path: /dev/foo
devices: null
kind: example.com/class

Which does not maintain ordering of the fields as defines in the spec.

The JSON output is as follows:

{"cdiVersion":"v0.3.0","kind":"example.com/class","devices":null,"containerEdits":{"deviceNodes":[{"path":"/dev/foo"}]}}

[elezar]

@klihub @bart0sh what are youre thoughts here? I'm willing to concede that this isn't too important, but reading generated specs becomes cumbersome expecially since the kind information is at the end of the file.

[klihub]

And is the order even stable ? I think it might not be. If we can find a non-intrusive way of getting a desired ordering, then it is worth a shot, but I agree that this is not too high on the list of priority-ordered TODO list.

[elezar]

I was under the impression that for json files the ordering of the fields is stable and defined by the order in which they are defined in a struct. If one uses the standard go YAML library, then this is also the case, but it requires that one add explict YAML tags to the structs definitions.

[elezar]

I have created #236 as an example of the required changes.

[bart0sh]

I think it's quite important for people who prefer to use yaml. Preserving an order is much more readable.
Please, go ahead with #236.