Add new 0.6 Knative Serving spec to generate command

[alexellis]

Expected Behaviour

Add new 0.6 Knative Serving spec to generate command

Current Behaviour

Compatible with the Alpha variant.

Possible Solution

Copy/paste code to output new format when --api flag matches the new beta API version

See also: generate.go

Steps to Reproduce (for bugs)

See also examples of the alpha generate spec #638 #637

[mattmoor]

If this is simply for generating the objects at creation, then the old fields will work fine (and are the most portable for clients to use). Adding v1beta1 support (when we expose the endpoint) should be doable just through calling ConvertUp on the types v1alpha1.Service (or other) resources, passing in an empty v1beta1.Service resource.

If you support manipulating the resources after they've been created then we should look more deeply into this now, otherwise we should just wait until this can start speaking to a v1beta1 endpoint for real (likely 0.8).