luci-base: sys.lua: lazy-load luci.model.network
luci-base: sys.lua: lazy-load luci.model.network

This allows using luci.sys without installed luci.model.network class.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

lbthomsen/openwrt-luci,openwrt/luci,rogerpueyo/luci,hnyman/luci,nmav/luci,nmav/luci,tobiaswaldvogel/luci,nmav/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,openwrt/luci,lbthomsen/openwrt-luci,openwrt/luci,nmav/luci,lbthomsen/openwrt-luci,openwrt/luci,hnyman/luci,hnyman/luci,hnyman/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,hnyman/luci,tobiaswaldvogel/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,rogerpueyo/luci,nmav/luci,hnyman/luci,nmav/luci,openwrt/luci,nmav/luci,lbthomsen/openwrt-luci,hnyman/luci,rogerpueyo/luci,openwrt/luci,rogerpueyo/luci,rogerpueyo/luci,tobiaswaldvogel/luci,nmav/luci,nmav/luci,hnyman/luci,openwrt/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,openwrt/luci,rogerpueyo/luci