Disable service links via defaults cm

[vagababov]

Permit disabling service links for the knative services

For #8498

/assign mattmoor

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-serving-unit-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-knative-serving-unit-tests:

pkg/apis/config.TestDefaultsConfigurationFromFile
pkg/apis/config.TestStoreLoadWithContext
pkg/apis/config.TestStoreLoadWithContextOrDefaults
pkg/apis/config.TestStoreImmutableConfig
pkg/reconciler/revision/config.TestStoreLoadWithContext
pkg/reconciler/revision/config.TestStoreImmutableConfig

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-serving-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/config/defaults.go	92.6%	93.9%	1.3

[mattmoor]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattmoor, vagababov

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Needs approval from an approver in each of these files:

• config/OWNERS [mattmoor,vagababov]
• pkg/apis/OWNERS [mattmoor,vagababov]

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