Do not change dev name in LanItemsClass#Items via reference. bnc#869776

Moreover LanItems.Items[key]["ifcfg"] can be immutable, so it could
raise an exception.
Testsuite for getNetworkInterfaces
Refactoring: getNetworkInterfaces
Updated changelog
Merge pull request #181 from mchf/bnc869776-internal-error

Internal error when starting network settings