Add template and attributes for nscd.conf

This probably isn't an exhaustive set of knobs that one would want to tweak, but does include the following improvements:

- Allow for togging the cache for each record type (`passwd` / `hosts` / &c)
- Set the server-user to nscd on *EL distros, use nobody (vs root) on debian and ubuntu. This isn't the upstream default, but seems to work fine in testing.

Note that I'm using a novel-ish way of passing the node attributes to the template -- seems sane to me, but would appreciate feedback.