Triggers working via path, rather than host

[Harwayne]

Fixes #918

Proposed Changes

• Triggers working via path, rather than host.
  • This removes the need to create both K8s Services and Istio VirtualServices.

Release Note

Existing K8s Services and Istio VirtualServices that correspond to Triggers will not be 
deleted proactively. They are cleaned up by K8s Garbage Collection, after their owning 
Trigger has been deleted.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Harwayne

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[Harwayne]

/assign @akashrv

[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/eventing/v1alpha1/trigger_lifecycle.go	Do not exist	71.4%
pkg/broker/receiver.go	82.9%	84.9%	2.0
pkg/reconciler/v1alpha1/subscription/subscription.go	73.1%	73.9%	0.9
pkg/reconciler/v1alpha1/trigger/trigger.go	79.5%	86.7%	7.2

[akashrv]

/lgtm