Reorganize pipeline

- Coverage is run on its own stage

.gitlab-ci.yml

 stages:
   - build
   - test
-  - deploy
+  - coverage
 
 variables:
   CTEST_OUTPUT_ON_FAILURE: 1
@@ -9,7 +9,7 @@ variables:
 include:
   - local: '/.gitlab-ci/gcc8-seq-dbg.yml'
   - local: '/.gitlab-ci/gcc8-seq-release.yml'
-  - local: '/.gitlab-ci/clang8-seq-coverage.yml'
   - local: '/.gitlab-ci/clang8-seq-release.yml'
-  - local: '/.gitlab-ci/clang8-mpi-coverage.yml'
   - local: '/.gitlab-ci/clang8-mpi-release.yml'
+  - local: '/.gitlab-ci/clang8-seq-coverage.yml'
+  - local: '/.gitlab-ci/clang8-mpi-coverage.yml'

.gitlab-ci/clang8-mpi-coverage.yml

-build:clang8-mpi-coverage:
+coverage:clang8-mpi-coverage:
   image: localhost:5000/ubuntu_clang8_mpi
-  stage: build
+  stage: coverage
   script:
     - mkdir -p build/clang8-cov-mpi
     - cd build/clang8-cov-mpi

.gitlab-ci/clang8-seq-coverage.yml

-build:clang8-seq-coverage:
+coverage:clang8-seq-coverage:
   image: localhost:5000/ubuntu_clang8
-  stage: build
+  stage: coverage
   script:
     - mkdir -p build/clang8-cov
     - cd build/clang8-cov