Changes

debian-stretch-build: add missing respawn.sh

In I3ec86c8610b3b43d39ea8e3da444861d317ced4e the container-individual
respawn.sh has been replaced with a common one - but unfortuantely
missing to update the debian-stretch-build, which made (at least)
ttcn3-bts-test builds fail for two nights now.

Change-Id: I15dcfe317d7ba81b9df0b5ceef142f4eae1ef3f3