Changes

debian-stretch-jenkins: add apt-transport-https

Fix installing packages from https with debian-stretch-jenkins after we
switched the packages mirror to https://downloads.osmocom.org/packages.

Fix for:
  E: The method driver /usr/lib/apt/methods/https could not be found.

Related: SYS#5557
Change-Id: I9ece7ffb295d8b371a0000a91b9b6d0f7174535d