Promote WEBHOOK_NAME convention, use consistently across knative

[mattmoor]

Problem

Eventing defines and uses:

eventing/pkg/logconfig/config.go

Line 37 in 52597d1

func WebhookName() string {

Persona:

Contributor

Exit Criteria

Knative consistently defines/uses WEBHOOK_NAME.

Time Estimate (optional):

1 to move to PKG and cleanup eventing.
2-3 to cleanup the rest of Knative.

Additional context (optional)

This is probably also relevant to Tekton cc @afrittoli

[arghya88]

@mattmoor should we move this to https://github.com/knative/pkg/blob/master/webhook/env.go ?

[mattmoor]

Yeah, that SGTM

[arghya88]

/assign