Remove cluster scoping of `ClusterIngress`

[tcnghia]

In what area(s)?

/area API
/area networking

Describe the feature

The work done #1997 allows us to avoid traffic splitting to a different namespace in ClusterIngress and opens the possibility to remove the namespace scoping of ClusterIngress. (Once we done that we can run Knative controllers to watch different namespaces, as well as restricting ACLs of Knative controllers to some specific namespaces only).

Thanks @wtam2018 for leading the charge on this one.

[tcnghia]

/assign @wtam2018

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Details

In response to this:

/assign @wtam2018

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[wtam2018]

Proposed staging/migration plan:
Release 0.x: Namespaced Ingress will be enabled which means that routes (new or existing) will create Namespaced Ingresses. Existing ClusterIngress and Virtual Services will continue to reconcile and run along side with Namespaced Ingresses.
Release 0.x+1: ClusterIngress resources will be deleted up and leaving Namespaced Ingresses active
Release 0.x+2: ClusterIngress will be cleaned up from source.