utils.NiceSort: Use sorted(), add keyfunc, unittests

This patch changes utils.NiceSort to use the built-in “sorted()” and
gets rid of the intermediate list. Instead of wrapping the items
ourselves, a key function is used. The caller can specify another key
function (useful to sort objects by their name, e.g.
“utils.NiceSort(instances, key=operator.attrgetter("name"))”.

Unittests are provided.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>