port assignment sucks

[elemoine]

The way we assign ports to MapFish apps sucks. Instead of defining a range of ports we should rely on port 0 and let the system pick a port of us. But after starting a MapFish app we need to know the port number that MapFih app uses (to give the user the app URL), so far I haven't found any clean way to get that information, it seems to me that we should not rely on subprocess to launch "paster serve" but do our own fork, use the PasteScript API and read the port from the server object returned by the API (needs investigation).