Implementation of LanItemsClass#delete_dev

This method removes cached configuration from internal caches of
LanItemsClass even underlying NetworkInterfaces (so deletes config from
system)
Testsuite for LanItemsClass#delete_dev
inst_setup_dhcp: clean also LanItems' cache when deleting. bcn#868830
Updated changelog
Testsuite for LanItemsClass#DeleteItem
Refactoring: LanItemsClass#DeleteItem