Remove cert-manager controller from default serving.yaml install

[RichieEscarez]

In what area(s)?

/area API
/area autoscale
/area build
/area monitoring
/area networking
/area test-and-release

What version of Knative?

HEAD

Expected Behavior

No new user blockers and optimal "first-touch" experience. Less "notes" and extra install related steps/info that are unnecessary for new users or others just wanting to "kick the tires"

Actual Behavior

The Knative Serving installation requires a --selector flag during install to prevent the
cert-manager controller from being installed by default.

https://github.com/knative/serving/releases/download/v0.5.2/serving.yaml --selector networking.knative.dev/certificate-provider!=cert-manager

Issue: If you do not immediately install cert-manager feature and CRDs, the cluster complains to your logs (not sure what issues that causes, ie is there a max log file size, and when the user becomes aware if they choose not to include the flag during install).

Example: https://github.com/knative/docs/blob/master/docs/install/Knative-custom-install.md

Due to the fact that HTTPS requires several prerequisite steps (ie. a domain, configuring that custom domain, install cert-manager, obtaining a TLS certificate, turning on HTTPS), it is unrealistic for users to perform this step immediately after a clean install, let alone a new user.

Since enabling HTTPS is an advanced steps, the cert-manager controller should not be installed by default, nor should our installation throw errors unnecessarily.

Steps to Reproduce the Problem

Run:

kubectl apply https://github.com/knative/serving/releases/download/v0.5.2/serving.yaml

Get: errors in logs

[RichieEscarez]

/assign @tcnghia

[mattmoor]

With ko-build/ko#46 we should be able to tease apart the release yamls so that we don't need to use the selector in our docs.

[jonjohnsonjr]

Pinging this since that PR got merged into ko.

[ZhiminXiang]

I created a tracking issue in knative/doc to track the work of updating installation instructions accordingly. knative/docs#1614