feat: make the server config file mode configurable (#561)

* feat: make the server config file mode configurable

/etc/rancher/k3s/config.yaml holds the cluster token. The server and
upgrade roles write it at 0644, so every local account on a server node
can read the credential that joins a node to the cluster.

k3s_server_config_mode sets the mode of that file in both roles. It
defaults to 0644, so behaviour is unchanged unless it is set.

Signed-off-by: Kristofer <kristofer@atlascloud.is>

* docs: describe the server config file contents accurately

/etc/rancher/k3s/config.yaml does not always carry the cluster token. A
first server that lets K3s generate one writes the file without it, so the
file can contain the token rather than always holding it. Correct that
claim in the README, the inventory sample and the role default, and use
the US spelling of "honors".

Signed-off-by: Kristofer <kristofer@atlascloud.is>

* Name the floor the config file mode can be narrowed to

Saying any mode root can read works leaves the reader to work out what
that means in practice. The file is read only by k3s, running as root, so
0600 is the tightest mode that still works.

Signed-off-by: Kristofer <kristofer@atlascloud.is>

* Run the basic integration job with a non-default config file mode

The templated mode was only ever rendered with its default in CI, which
is the one value that cannot tell a working template from a broken one.
The basic inventory sets 0640 and the job reads the mode back off the
installed file.

Signed-off-by: Kristofer <kristofer@atlascloud.is>

---------

Signed-off-by: Kristofer <kristofer@atlascloud.is>

inventory-sample.yml

@@ -34,6 +34,7 @@ k3s_cluster:
     # extra_service_envs: [ 'ENV_VAR1=VALUE1', 'ENV_VAR2=VALUE2' ]
     # user_kubectl: true, by default kubectl is symlinked and configured for use by ansible_user. Set to false to only kubectl via root user.
     # manage_firewall: false
+    # k3s_server_config_mode: "0644", mode of /etc/rancher/k3s/config.yaml on server nodes. The file can contain the cluster token; set "0640" to keep it off other local accounts.
 
     # Manifests or Airgap should be either full paths or relative to the playbook directory.
     # List of locally available manifests to apply to the cluster, useful for PVCs or Traefik modifications.