Implement containerd environment variables whitelisting

[skgsergio]

Currently, when using cadvisor with containerd it exposes all the environment variables for every container which can be dangerous (i.e. secret env vars in k8s) and can result in big and slow metrics responses (/metrics endpoint in our environment takes some seconds to reply and gives a >500MB body size) and high memory usage.

This PR adds the whitelisting functionality mimicking the same functionality in the docker module to keep consistency between them.

[k8s-ci-robot]

Hi @skgsergio. Thanks for your PR.

I'm waiting for a google 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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[iwankgb]

/ok-to-test