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
Added comments to SetItemName, SetCurrentName
Merge pull request #173 from mchf/bnc868859-delete-item

When deleting dev config in inst_setup_dhcp, update also LanItems::Items