internal/envoy: Refactor httpConnectionManagerBuilder

[stevesloka]

The httpConnectionManagerBuilder in internal/envoy/listener.go needs some love. Many of the builder commands aren't optional and should be refactored to always be present.

Additionally, we shouldn't use the same builder helper methods in the tests to validate the results, we're essentially just doing a no-op test that validates our helpers work, not that we're producing proper output to Envoy.

Originally posted by @stevesloka in #2734 (comment)

_Slack Thread: https://kubernetes.slack.com/archives/C8XRH2R4J/p1595886040392900

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[github-actions]

The Contour project currently lacks enough contributors to adequately respond to all Issues.

This bot triages Issues according to the following rules:

• After 60d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, the Issue is closed

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• Mark this Issue as fresh by commenting
• Close this Issue
• Offer to help out with triage

Please send feedback to the #contour channel in the Kubernetes Slack