Adding the action to the patterns if there are no descriptors

Author: kobelb

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/privilege/ApplicationPrivilege.java

@@ -193,7 +193,7 @@ private static ApplicationPrivilege resolve(String application, Set<String> name
                 if (descriptor != null) {
                     patterns.addAll(descriptor.getActions());
                 } else {
-                    throw new IllegalArgumentException("unknown application privilege [" + name + "]");
+                    patterns.add(name);
                 }
             } else {
                 actions.add(name);