configfile: cache unique keys based on opts
Merge pull request #799 from msimerson/cfreader-cache

configfile: cache unique keys based on opts
server: corrected defaults applying
fixes spurious test error in PR #796

server.js was throwing an error, but only when run *after* the
net_utils tests. I tracked it down to this line causing the problem:
```js
config.get('smtp.ini').main.public_ip
```

This ends up being an issue related to config caching. `net_utils`
reads a copy of smtp.ini to see if a public IP is defined there but it
does’t pass in the same options as the server.js file. Later, when
server.js reads that same file, with options, it gets the cached result
without them.
net_utils: remove no_cache option