When mgmt is in etcd-client-only mode and using an external etcd server,
we don't want to unset our only known endpoint since this would deadlock
our etcd client since it can't connect to anyone. This could have
happened because a plain etcd server didn't set any endpoints to follow,
and as a result we noticed it was empty and decided to use that instead.
To workaround this issue on an earlier version of mgmt, you would have
had to run:
etcdctl put /_mgmt/endpoints/etcd http://localhost:2379
to set this magic key on the initial etcd server.
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