GenericCheck: nested field match like %(role.name)s does not work without flattening dict

The examples of GenericCheck shows like:

        - True:%(user.enabled)s
        - 'Member':%(role.name)s

i think these examples expect that 'target' contains a field like "user.enabled" or "role.name", but in most cases we don't have such fields without flattening "target" dict.

keystone has a convenient method to flatten a dict [1] and and it is called before policy enforcement (e.g., [2]).

It would be nice and useful if oslo.policy supports flattening a target dict before policy.check.
Otherwise, examples of GenericCheck should be updated not to include nested fields.

[1] https://github.com/openstack/keystone/blob/master/keystone/common/utils.py#L72-L86
[2] https://github.com/openstack/keystone/blob/master/keystone/common/controller.py#L161