change to vector to allow brace-enclosed initializer list in tests

Closes #2401 (closed).

There's no reason for the address list to be a multi index container, it is not even mocking that container. It can be a simple vector.

Test run: https://jenkins.aws.isc.org/view/Kea-manual/job/kea-manual/job/ut-basic/1/execution/node/107/log/

It passed the point of error. It fails on another error. That is caused by another issue.