Changes

debian-stretch-build-dist: use -obs-latest

Change debian-stretch-build-dist to be based on
debian-stretch-obs-latest instead of debian-stretch-build. The latter
installs the nightly OBS repository now (as that is what we need for the
TTCN-3 builds using debian-stretch-build), but debian-stretch-build-dist
needs to install packages from OBS latest.

Fixes jenkins failures:
  The following packages have unmet dependencies:
   libasn1c-dev : Depends: osmocom-latest but it is not going to be installed
   ...

Related: OS#5049
Change-Id: I2cf6ed732e55b8f0e63bd8fc040749bd16676d0f