Update pcep-topology-type-config placement

We are currently doing this in network-pcep-topology-config.yang:

    augment "/nt:network-topology/nt:topology" {
        when "nt:topology-types/pn:topology-pcep";
        ext:augment-identifier pcep-topology-type-config;
        description "Augment Pcep topology with session config";

        uses pdc:pcep-config;
    }

This adds the configuration as an optional thing, where as we really want to make it required – which means it should be part of network-topology-pcep.yang's 'topology-pcep'.

Since topology-pcep is a presence container, it forms a mandatory leaf validation root, and therefore we can express exactly what we mean: things like listen-address etc. are mandatory and it is invalid to try to have a pcep-topology without those knobs.

This will simplify a lot of logic, as we can rely on datastore to enforce this invariant.