Debian 'jessie' even with backports lacks some packages Kolla expects. Due
to this we (Linaro) move to Debian 'stretch' release with our OpenStack
work.
Stretch is now frozen so there should not be many changes.
Partially-Implements: blueprint multiarch-and-arm64-containers
Change-Id: Iab7232cbe6f8fc3e3de10c67f3d89f134b185f05