Changes

osmo-gsm-tester: Require debian-stretch-jenkins as Dockerfile suggests
Some days ago the Dockerfile was changed to pull from 
debian-stretch-jenkins instead of debian-stretch-build, since the first 
one has extra runtime dependencies required by osmo-gsm-tester. However,
the jenkins script is still requiring the old dependency. Let's update
it accordingly.
Change-Id: If33aab60e04da7beb4cd0715d5cb3145c001823a