Allow setting catch all virtual hosts

It would be really useful if I didn't have to set a virtual host. On EC2 I have to set the virtual host to a whole load of values because:
1. On ELB health checks the Host header is the instance private IP address
2. On normal requests the Host header is the domain

http://serverfault.com/questions/527156/setting-nginx-to-catch-all-unhandled-vhosts
