Rewrite nginx.conf files based on latest Docker dynamic port mappings on server reboot

[eugeneware]

When a server reboots all the dynamic port mappings from Docker get reset leading to 502 bad gateway errors in Nginx, or even worse: the wrong servers displaying for different host names.

This PR addresses the issue in #82

It creates a new upstart service called dokku.conf which is run after docker that will iterate through the dokku apps, find the CONTAINER IDs, find the port mapping, rewrite the nginx.conf files and then reload nginx.

[eugeneware]

Most PaaS have some sort of routing layer to map from external ports to internal container ports.

Another alternative to rewriting the nginx.conf ports on reboot would be to use similar logic above, but do the port map dynamically upon reboot - see: http://wiki.nginx.org/HttpMapModule

Upon reboot we could scan the CONTAINER IDs and ports, and then dynamically generate a single mapping file that gets included by nginx to do the port mapping.

Does this sound like a valid alternative?

The team at docker haven't been very responsive to my enquiries about the dynamic port allocations.

[ghost]

Merged #178

[asm89]

Untill we have a more solid approach with a component that's in the working for flynn, we decided to go with the approach in #178. Can you check it out if it works for you too?