[master] Fix a metrics module installation bug when overlay network is disabled.

[greggomann]

High-level description

This PR fixes an issue in dcos-config.yaml which breaks the metrics Mesos module when the overlay network has been disabled at install time.

Corresponding DC/OS tickets (obligatory)

These DC/OS JIRA ticket(s) must be updated (ideally closed) in the moment this PR lands:

• DCOS_OSS-4521 Metrics Mesos module fails to load if overlay network is disabled

Checklist for all PRs

• Added a comprehensible changelog entry to CHANGES.md or explain why this is not a user-facing change:
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• Read the DC/OS contributing guidelines
• Followed relevant code rules Rules for Packages and Systemd

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[greggomann]

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Your pull request's branch is not based on the most recent version of master. Please rebase your changes against this repo's master branch.

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Enterprise Bump PR: mesosphere/dcos-enterprise/pull/3841

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Enterprise Bump mesosphere/dcos-enterprise/pull/3841 updated.

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