Add NixOS module to control power policy

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/module/power-policy.nix

@@ -0,0 +1,31 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.power.policy;
+in
+{
+  options = {
+    power.policy = mkOption {
+      type = types.nullOr (types.enum [ "always-on" "previous" "always-off" ]);
+      default = null;
+      description = "Set power policy to use via IPMI.";
+    };
+  };
+
+  config = mkIf (cfg != null) {
+    systemd.services."power-policy" = {
+      description = "Set power policy to use via IPMI";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        ExecStart = "${pkgs.ipmitool}/bin/ipmitool chassis policy ${cfg}";
+        Type = "oneshot";
+        Restart = "on-failure";
+        RestartSec = "5s";
+        StartLimitBurst = "10";
+        StartLimitIntervalSec = "10m";
+      };
+    };
+  };
+}