prometheus-node-exporter-lua: add openwrt exporter

prometheus-node-exporter-lua: add openwrt exporter

adds openwrt specific information about the device.
include DISTRIB_{ID, RELEASE, REVISION}, board_name and model

Example output:

    # TYPE node_openwrt_info gauge
    node_openwrt_info{revision="55a0636",model="QEMU Standard PC (i440FX + PIIX, 1996)",id="LiMe",board_name="qemu-standard-pc-i440fx-piix-1996",release="snapshot"} 1
    node_scrape_collector_duration_seconds{collector="openwrt"} 3.814697265625e-05
    node_scrape_collector_success{collector="openwrt"} 1

Signed-off-by: Paul Spooren <spooren@informatik.uni-leipzig.de>