gitlab-ci: run unit tests under valgrind in gitlab-ci

On Ubuntu 16.04 (trusty) valgrind fails due to rdrand being advertised
but not implemented.

Work around that by installing valgrind from Ubuntu 18.04 (bionic) via
the "contrib/scripts/nm-ci-install-valgrind-in-ubuntu1604.sh" script.
12 jobs for th/gitlab-ci-valgrind in 39 minutes and 42 seconds (queued for 2 seconds)