Rename traffic split 'name' to 'subdomain'

[whaught]

Spec Issue

When setting up a traffic split in a Route, we assign a traffic split a 'name'. We use this field to determine where revisions should serve by prepending the name as a subdomain of the route's URL.
However we do not use this in any other way to address the split as a named object or as an identifier.

It should be called 'subdomain' for clarity. That's what it does, that's what it is.

Example

apiVersion: knative.foo.com/v1alpha1
kind: Route
metadata:
  name: my-service
spec:
  rollout:
    traffic:
    - revisionName: phi
      name: **currentRelease**
      percent: 75
    - revisionName: eta
      name: **nextRelease**
      percent: 25 
    - configurationName: rho
      name: **testing123**
      percent: 0 

If we serve at "foo.com", we expect "currentRelease.foo.com", "nextRelease.foo.com" and "testing123.foo.com" to serve individual revisions.

name would be more apt as subdomain

[mikehelmick]

related to release mode in #1865

[evankanderson]

For compatibility, we will probably need to keep both name and subdomain until we have the ability to transform CRDs between versions on the server side. (I think @jonjohnsonjr is tracking this, but it is unlikely before k8s 1.13.)

If we're okay with having both and enforcing (via webhook) that they don't diverge, this name change seems plausible to me. Would like @jonjohnsonjr or @mattmoor to take a look too, in case I'm missing something.

[mattmoor]

I don't have any real objection to this, other than it's going to be a glorious pain to do before mid next year due to the K8s slips in CRD versioning.

cc @tcnghia

[mattmoor]

Going to put this in v1, and we should consider this as part of the v1 API discussion, but if/how we rename it TBD.

[mattmoor]

We've renamed this tag for v1beta1.