Allow an unknown agent to be configured in prow

[wbrefvem]

This PR is intended to address issues raised in the review of #13199. Instead of returning an error when an unknown agent is configured, this change causes prow to emit a warning and return nil. This means that prow can support any agent the user cares to configure without having to know about all potential agents in advance.

cc @stevekuznetsov @fejta @ccojocar @abayer

[k8s-ci-robot]

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[wbrefvem]

/assign @stevekuznetsov

[matthyx]

/ok-to-test
I am not sure if this is a good idea... What are the implications of using an agent that is not expected?

[wbrefvem]

@matthyx My colleague @ccojocar had raised a PR that specifically added the agent we wanted, and the feedback we got from @stevekuznetsov was that it would be better to introduce a more permissive approach that allows a user to bring their own agent without having to request changes to prow.

[wbrefvem]

@matthyx As for the implications, it just means that an unknown agent a) won't get any validation and b) will be trusted, so far as that goes. The idea behind issuing a warning is to ensure that only users that know what they're doing do this.

[alvaroaleman]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3a6f721b14216172ae64df8317fd0253b02f4355

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: stevekuznetsov, wbrefvem

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