We don't need to set $TZ, because we have /etc/localtime. In fact,
setting $TZ without $TZDIR doesn't work anymore since Glibc no longer
contains zone info.
This allows hiding the implementation details for how to represent logstash
config types that don't directly map to nix expressions, particularly floats,
hashes, and name-value pair sets with repeated names. Instead of setting
__type and value directly, the user now uses these convenience functions to
generate their logstash config.
Since the logstash config file seemed very similar to a nixexpr, I decided
to map directly from nixexprs to logstash configs. I didn't realize until
too far in that this solution was probably way over-engineered, but it
works.
