Just a quick note.
Nginx will include configuration files based on ASCII ordering. This means that even if you have two files default (with the default config) and cloud, cloud will be included before default and will answer for any undefined hostnames.
The easy way to solve this (and obvious if you have been around Linux for a while) is to prefix all config files with a number. For example:
00-default 10-wildcard.example.com 10-wildcard.foo.com
This will result in 00-default being included in the configuration first.
I hope this is useful for someone out there.