Added // restore // guide to single-to-multi-master.md

[vlaza]

In case single to multi-master migration fails and etcd become inconsistent you can use the revert guide.

[k8s-ci-robot]

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[vlaza]

/assign @justinsb

[vlaza]

/retest

[k8s-ci-robot]

@vlaza: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[chrisz100]

/ok-to-test
/lgtm
/area documentation

Thank you for the contribution!

[vlaza]

/retest

[chrisz100]

/retest

[mikesplain]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrisz100, mikesplain, vlaza

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[vlaza]

/test pull-kops-e2e-kubernetes-aws