KIngress should disallow ServiceNamespace that differs from its own

[mattmoor]

At present the validation just checks that it is specified:

serving/pkg/apis/networking/v1alpha1/ingress_validation.go

Lines 138 to 140 in ce240ab

	if b.ServiceNamespace == "" {
	all = all.Also(apis.ErrMissingField("serviceNamespace"))
	}

One of the fundamental differences between ClusterIngress and Ingress was that the former allowed this and the latter should disallow this.

I noticed this by accident playing with custom HTTP01 challenges, and we should plug this hold before someone takes a dependency on this, since it is not supported by all KIngress implementations.

/kind good-first-issue

[mattmoor]

cc @tcnghia @wtam2018

[markusthoemmes]

/good-first-issue

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[MIBc]

I want to try.

[markusthoemmes]

/assign @MIBc