Support etcd-manager v3, suitable for backporting

[justinsb]

Add etcd-manager v3 in a way that we can safely backport.

[mikesplain]

Thanks for making it backport-able! I think that's a good path forward to migrate people to etcd3!

Thanks!

/retest
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: justinsb, mikesplain

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[ottoyiu]

@justinsb @mikesplain what does it mean to be 'backportable'? it's not abundantly clear what it means from looking at the code. Does that mean that users of kops can fallback to not use etcd-manager?

ie. I can use etcd-manager to upgrade from etcdv2 to etcdv3 and then fallback to directly using etcdv3 using protokube/r53 records?

[zetaab]

/test pull-kops-e2e-kubernetes-aws