Allow configuring augerctl using ETCDCTL env vars

Load certs and endpoints from ETCDCTL env vars as a fallback:

```
export ETCDCTL_CACERT=/etc/etcd/pki/ca.crt
export ETCDCTL_CERT=/etc/etcd/pki/client.crt
export ETCDCTL_KEY=/etc/etcd/pki/client.key
export ETCDCTL_ENDPOINTS=https://127.0.0.1:2379
```

Happy to provide a PR if you are interested.