Propagate labels and annotations from Route to placeholder service

[nak3]

This patch changes to propagate placeholder service's labels and
annotations from Route.

Currently there are no way to add annotation and labels to the
placeholder service except for manual.

/lint

Release Note

Placeholder service's labels and annotations are propagated from Route.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nak3

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[knative-prow-robot]

@nak3: 0 warnings.

In response to this:

This patch changes to propagate placeholder service's labels and
annotations from Route.

Currently there are no way to add annotation and labels to the
placeholder service except for manual.

/lint

Release Note

Placeholder service's labels and annotations are propagated from Route.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[nak3]

This is based on user's request: https://knative.slack.com/archives/CA9RHBGJX/p1595390943143500

And one note:
I didn't change it to reconcile the placeholder service aggressively.
It means that even if we updated the label to the route, the placeholder service is not reconciled automatically. It is the same behavior with current KIngress.

[vagababov]

/lgtm

[yanweiguo]

/lgtm

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-serving-integration-tests	2020-07-24 19:00:44.355 +0000 UTC	1/3

Automatically retrying due to test flakiness...
/test pull-knative-serving-integration-tests