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/config.yaml

@@ -3,10 +3,6 @@
 # The service via which Nixery is exposed has a private DNS entry
 # pointing to it, which makes it possible to resolve `nixery.local`
 # in-cluster without things getting nasty.
-#
-# The 'nixery-keys' secret was configured manually using a created
-# service account key. This does not use metadata-based authentication
-# due to the requirement for having an actual PEM-key to sign with.
 ---
 apiVersion: apps/v1
 kind: Deployment