Avoid YAML serialization inside `Deployer.discover_node_configuration`.
Add an API for discovering used ports to `INetwork`.
Implement ``INetwork.enumerate_used_ports`` on ``HostNetwork``.
Add a docstring for that implementation.
Add a test for ``HostNetwork`` noticing ports bound in the usual way - by a server or a client.
Add `used_ports` to `NodeState` and have `Deployer.discover_node_configuration` populate it.