feat(k8s): Insert Nixery's secrets via kontemplate

Instead of having a manually prepared secret, use Cloud KMS (as per
the previous commits) to decrypt the in-repo secrets and template them
into the Secret resource in Kubernetes.

Not all of the values are actually secret, it has thus become a bit
easier to edit the known hosts, SSH config and such now.

infra/kubernetes/nixery/secrets.yaml

@@ -0,0 +1,19 @@
+# The secrets below are encrypted using keys stored in Cloud KMS and
+# templated in by kontemplate when deploying.
+#
+# Not all of the values are actually secret (see the matching)
+---
+apiVersion: v1
+data:
+  gcs-key.json: {{ passLookup "nixery-gcs-json" | b64enc }}
+  gcs-key.pem: {{ passLookup "nixery-gcs-pem" | b64enc }}
+  id_nixery: {{ passLookup "nixery-ssh-private" | b64enc }}
+  id_nixery.pub: {{ insertFile "id_nixery.pub" | b64enc }}
+  known_hosts: {{ insertFile "known_hosts" | b64enc }}
+  ssh_config: {{ insertFile "ssh_config" | b64enc }}
+kind: Secret
+metadata:
+  creationTimestamp: null
+  name: nixery-secrets
+  selfLink: /api/v1/namespaces/kube-public/secrets/nixery-secrets
+type: Opaque