Allow fastforward to run non-interactive

[saschagrunert]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Recent runs of the fast-forward jobs are failing because we have no non-interactive flag for the GCB job run. We now fix that by making the question optional.

Example: https://storage.googleapis.com/kubernetes-jenkins/logs/ci-fast-forward/1522088732356775936/build-log.txt

Which issue(s) this PR fixes:

Refers to #2386

Special notes for your reviewer:

Gave it a test run in https://console.cloud.google.com/cloud-build/builds/c601bfb8-bf5e-4934-86f7-6f2165379823

Does this PR introduce a user-facing change?

Added `NonInteractive` flag to gcb options to allow asking no questions when running in nomock mode.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: saschagrunert

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• pkg/OWNERS [saschagrunert]

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[ameukam]

/lgtm