[!248] Centralize CI configuration

Merge branch 'ci/centralize-config' into 'master'

See merge request [!248]

  [!248]: Nonecore/dune-istl/merge_requests/248

.gitlab-ci.yml

 ---
+include:
+  - "https://gitlab.dune-project.org/core/ci-config/raw/master/config/common/master.yml"
+  - "https://gitlab.dune-project.org/core/ci-config/raw/master/jobs/common/master.yml"
+
 before_script:
   - duneci-install-module https://gitlab.dune-project.org/core/dune-common.git
 
@@ -10,31 +14,3 @@ variables:
 # number of cores. We solve the issue by disallowing OpenMP to allocate more
 # than one thread.
   OMP_NUM_THREADS: 1
-
-debian:10  gcc:c++17:
-  image: duneci/base:10
-  script: duneci-standard-test
-  variables: {DUNECI_OPTS: /duneci/opts.gcc.c++17}
-  tags: [duneci]
-
-debian:9--gcc:
-  image: duneci/base:9
-  script: duneci-standard-test
-  tags: [duneci]
-
-debian:9--clang:
-  image: duneci/base:9
-  script: duneci-standard-test
-  variables: {DUNECI_OPTS: /duneci/opts.clang}
-  tags: [duneci]
-
-ubuntu:16.04--gcc:
-  image: duneci/base:16.04
-  script: duneci-standard-test
-  tags: [duneci]
-
-ubuntu:16.04--clang:
-  image: duneci/base:16.04
-  script: duneci-standard-test
-  variables: {DUNECI_OPTS: /duneci/opts.clang}
-  tags: [duneci]