Allow preprepared CoreDNS image to be used

[aaronjwood]

What type of PR is this?
/kind bug
/area conformance-machinery

What this PR does / why we need it:
For folks that are pulling images from Docker's public registry in an unauthenticated way rate limits are an immediate problem. This PR fixes two things:

1. Explicitly sets the pull policy for CoreDNS to be IfNotPresent, allowing for a cluster with an already-prepared CoreDNS image to be used
2. Fixes getContentsFromPathOrURL to correctly handle multiple fs.FS. If this was already supported client code could've easily done something like this to work around the issue:

virtFS := fstest.MapFS{...} // Imagine this to hold an override for the manifest with issues.
suite.ConformanceOptions{
...
    ManifestFS:               []fs.FS{&conformance.Manifests, &virtFS},
...
}

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

@aaronjwood: The label(s) kind/conformance-machinery cannot be applied, because the repository doesn't have them.

In response to this:

What type of PR is this?
/kind bug
/kind conformance-machinery

What this PR does / why we need it:
For folks that are pulling images from Docker's public registry in an unauthenticated way rate limits are an immediate problem. This PR fixes two things:

1. Explicitly sets the pull policy for CoreDNS to be IfNotPresent, allowing for a cluster with an already-prepared CoreDNS image to be used
2. Fixes getContentsFromPathOrURL to correctly handle multiple fs.FS. If this was already supported client code could've easily done something like this to work around the issue:

virtFS := fstest.MapFS{...} // Imagine this to hold an override for the manifest with issues.
suite.ConformanceOptions{
...
   ManifestFS:               []fs.FS{&conformance.Manifests, &virtFS},
...
}

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: aaronjwood
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• conformance/OWNERS

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @aaronjwood. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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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-sigs/prow repository.

[aaronjwood]

/area conformance-machinery

[mikemorris]

/ok-to-test

[aaronjwood]

The failing test looks like some kind of flake...? Is it known to anybody?

[aaronjwood]

/retest

[mikemorris]

I missed which test was failing, looks like everything is in good shape now.

Tagging conformance leads for review @candita @sunjayBhatia @arkodg @LiorLieberman