Add a new method like as_dict that returns labels with juju_ prefix

[Abuelodelanada]

Enhancement

In juju_topology.py we should add a new method like .as_dict that returns the labels prefixed with juju_.

Without this method we have to do things like these:

                        # TODO: We should add a new method in juju_topology.py
                        # that like `as_dict` method, returns the keys with juju_ prefix
                        **{
                            "juju_{}".format(key): value
                            for key, value in topo_as_dict.items()
                            if value
                        },