FIX: kube-apiserver config only has single etcd server endpoint (#179)

**Before**
```
spec:
  containers:
  - command:
    - kube-apiserver
    - ...
    - --etcd-servers=https://ip-10-1-2-3.k8s.cluster:2379
```
**After**
```
spec:
  containers:
  - command:
    - kube-apiserver
    - ...
    - --etcd-servers=https://ip-10-1-2-3.k8s.cluster:2379,https://ip-10-4-5-6.k8s.cluster:2379,https://ip-10-7-8-9.k8s.cluster:2379
```
```

modules/aws/kube-etcd/outputs.tf

@@ -1,6 +1,6 @@
 output "endpoints" {
   value = [
-    for etcd_ip in aws_network_interface.etcd.*.private_ips[0] :
+    for etcd_ip in flatten(aws_network_interface.etcd.*.private_ips) :
     "https://ip-${replace(etcd_ip, ".", "-")}.${local.discovery_service}:${local.client_port}"
   ]
 }