Add check for the state parameter in manifest.

This only allow following cases:
  - present
  - absent
else, it will abort with an error message.

cdist/conf/type/__cron/manifest

@@ -22,3 +22,12 @@ if [ -f "$__object/parameter/raw" ] && [ -f "$__object/parameter/raw_command" ];
     echo "ERROR: both raw and raw_command specified" >&2
     exit 1
 fi
+
+case "$(cat "$__object/parameter/state" 2>/dev/null || echo "present")" in
+    present) ;;
+    absent) ;;
+
+    *)
+        echo "ERROR: unkown cron state" >&2
+        exit 2
+esac