Perform preemption check only when used as a chained plugin

[luksa]

When Istio CNI is not used as a chained plugin, throwing an
error or logging the preemption warning doesn't make sense.
For example, when using Multus CNI, multiple CNI config
files are in the same dir, but none of them has precedence
over the others, as they are loaded by name. In that case,
logging the warning isn't appropriate.

[istio-policy-bot]

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[istio-testing]

Hi @luksa. Thanks for your PR.

I'm waiting for a istio 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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[jwendell]

/ok-to-test