feat(zfs): add declarative management for ZFS datasets and integration tests

common/global/snapshot_manager.nix

@@ -22,6 +22,11 @@ in
           the PYTHONPATH to use for the snapshot_manager service.
         '';
       };
+      package = lib.mkOption {
+        type = lib.types.package;
+        default = pkgs.my_python;
+        description = "Python environment used to run snapshot_manager.";
+      };
       EnvironmentFile = lib.mkOption {
         type = lib.types.nullOr (lib.types.coercedTo lib.types.path toString lib.types.str);
         default = null;
@@ -45,7 +50,7 @@ in
         };
         serviceConfig = {
           Type = "oneshot";
-          ExecStart = "${pkgs.my_python}/bin/python -m python.tools.snapshot_manager ${lib.escapeShellArg cfg.path}";
+          ExecStart = "${cfg.package}/bin/python -m python.tools.snapshot_manager ${lib.escapeShellArg cfg.path}";
         }
         // lib.optionalAttrs (cfg.EnvironmentFile != null) {
           EnvironmentFile = cfg.EnvironmentFile;