Without supplying the --enable-v2=true flag to etcd on startup, patroni cannot find etcd to run.
after running `etcd --data-dir=data/etcd` in one terminal and `patroni postgres0.yaml` in another terminal, etcd starts fine, but the postgres instance cannot find etcd.
```
patroni postgres0.yaml
2020-05-09 15:58:48,560 ERROR: Failed to get list of machines from http://127.0.0.1:2379/v2: EtcdException('Bad response : 404 page not found\n')
2020-05-09 15:58:48,560 INFO: waiting on etcd
```
If etcd is passed the flag `--enable-v2=true` on startup, everything works fine.
