Update .gitlab-ci.yml to handle centOS 8 which is the operating system of the maestro cluster.

Added checking that make distcheck works well.

.gitlab-ci.yml

 makefile_generation_and_compilation_plus_run_nrt_tests_cluster:
-    image: registry-gitlab.pasteur.fr/dsi-tools/docker-images:centos7_rock
+    image: registry-gitlab.pasteur.fr/dsi-tools/docker-images:centOS8_ROCK
     script: 
         - source /etc/profile.d/gensoft_modules.sh 
         - module av
@@ -23,6 +23,7 @@ makefile_generation_and_compilation_plus_run_nrt_tests_cluster:
         - ./unit_test_cms
         - ./unit_test_read_utils
         - cd ..; make check
+        - make distcheck