Avoid duplicate entries for the metadata server in /etc/hosts

[rjschwei]

In cases where the metadata server /etc/hosts entry was not added by the guest environment scripts there will be no "# Added by Google" marker. As such the script will add a second reference to the metadata if it already exists. We should only have 1 entry. Respect the already existing entry and do not add a second entry.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: rjschwei
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[google-oss-prow]

Hi @rjschwei. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform 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.

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[a-crate]

FYI, it will probably be several weeks before this can be merged and released due to holiday code and release freezes.