luci-app-statistics: fix collectd config generation

Fix config generation for the following sections:

* curl
* exec
* network
* iptables

Ref: https://github.com/openwrt/packages/issues/9427
Fixes: c1380ab3

 ("make luci-app-statistics more extensible")

Signed-off-by: Florian Eckert <fe@dev.tdt.de>