fix: reject reused scheme:host pairs in port_mappings

[josegonzalez]

Only one container port can be bound to a given scheme and host port, so dokku refuses a ports:add or ports:set whose mappings reuse one. dokku_ports accepted such a list and only failed once the command ran. Validate() now rejects a reuse within the recipe, which docket validate catches offline, and plan reports a mapping that collides with one already on the server instead of leaving it to apply. state: absent still accepts a reuse, matching ports:remove.

Closes #432