Change the Channelable interface to use URIs.

[adamharwayne]

Another piece of #542.

Proposed Changes

• Stop using the knative/pkg's Channelable duck type. Instead make our own in this repo.
  • I elected to continue with a duck Channelable, rather than just using eventing's Channel, because we have already done the work to make the duck portion work in the Subscription controller and believe that we might want this feature post 0.1.
• Change Channelable to use URIs, rather than domains. As specified in Adding Knative Eventing Object Model targeting 0.1 release. #482.

Release Note

You will need to rebuild and deploy:

1. Delete ALL existing Channels.
    kubectl get channels.eventing; kubectl delete channels.eventing ...
2. Any Channel ClusterProvisioners.
    kubectl get clusterprovisioners; kubectl delete clusterprovisioner ...
3. The main eventing controller.
    ko apply -f config/500-controller.yaml
4. The main eventing webhook.
    ko apply -f config/500-webhook.yaml
5. The Channel CRD.
    ko apply -f config/300-channeleventing.yaml

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/duck/v1alpha1/channelable_types.go	Do not exist	0.0%
pkg/controller/eventing/subscription/reconcile.go	77.8%	78.0%	0.3

[adamharwayne]

/assign @evankanderson

[evankanderson]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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