ns-gce is unable to join the cluster because its source IP address is
the node on which its running, 34.72.45.206, and that's not included in
the SANs.
This commit updates the etcd certificate to one which includes the three
GKE nodes' IP addresses in its SANs.
This commit also includes instruction to update the certificates in the
event of an IP address change.
Fixes:
```
Apr 16 14:15:34 ns-aws etcd[500]: rejected connection from "34.72.45.206:43080" (error "tls: \"34.72.45.206\" does not match any of DNSNames [\"ns-aws.sslip.io\" \"ns-azure.sslip.io\" \"ns-gce.sslip.io\" \"ns-aws\" \"ns-azure\" \"ns-gce\"] (lookup ns-gce: Temporary failure in name resolution)", ServerName "ns-aws.sslip.io", IPAddresses ["127.0.0.1" "52.0.56.137" "52.187.42.158" "104.155.144.4" "::1" "2600:1f18:aaf:6900::a"], DNSNames ["ns-aws.sslip.io" "ns-azure.sslip.io" "ns-gce.sslip.io" "ns-aws" "ns-azure" "ns-gce"])
```
