k0: enable fstrim, lower gc thresh for kubelet

fstrim is nice as it might prevent us from killing SSDs so fast.

A lower GC threshold for kubelet is nice as we run non-kubelet services
on these nodes, and they need their space. Notably, Ceph's mons tend to
be extremely claustrophobic, firing alerts at 70% disk usage or so.

Change-Id: I94c1787e62f82a02f107d04a87575327d3d79c01
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1724
Reviewed-by: implr <implr@hackerspace.pl>

cluster/machines/modules/base.nix

@@ -114,6 +114,13 @@ in {
   
     # Enable the OpenSSH daemon.
     services.openssh.enable = true;
+
+    # Enable fstrim, as we run / on SSDs everywhere.
+    services.fstrim = {
+      enable = true;
+      interval = "daily";
+    };
+
     users.users.root.openssh.authorizedKeys.keys = [
       "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDD4VJXAXEHEXZk2dxNwehneuJcEGkfXG/U7z4fO79vDVIENdedtXQUyLyhZJc5RTEfHhQj66FwIqzl7mzBHd9x9PuDp6QAYXrkVNMj48s6JXqZqBvF6H/weRqFMf4a2TZv+hG8D0kpvmLheCwWAVRls7Jofnp/My+yDd57GMdsbG/yFEf6WPMiOnA7hxdSJSVihCsCSw2p8PD4GhBe8CVt7xIuinhutjm9zYBjV78NT8acjDUfJh0B1ODTjs7nuW1CC4jybSe2j/OU3Yczj4AxRxBNWuFxUq+jBo9BfpbKLh+Tt7re+zBkaicM77KM/oV6943JJxgHNBBOsv9scZE7 q3k@amnesia"
       "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIG599UildOrAq+LIOQjKqtGMwjgjIxozI1jtQQRKHtCP q3k@mimeomia"

cluster/machines/modules/kube-dataplane.nix

@@ -80,7 +80,9 @@ in {
         clusterDns = "10.10.12.254";
         kubeconfig = pki.kube.kubelet.config;
         extraOpts = ''
-          --read-only-port=0
+          --read-only-port=0 \
+          --image-gc-high-threshold=60 \
+          --image-gc-low-threshold=40
         '';
         package = config.hscloud.kube.packageKubelet;
       };