Automatic CI for MRs

Issue-Ref: 4093
Signed-off-by: Alexander Diewald <diewald@fortiss.org>

.gitlab-ci.yml

+downstream-job:
+  variables:
+    BUILD_BRANCH: "$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"
+  only:
+    - merge_requests
+  trigger:
+    project: af3/maven-releng
+    strategy: depend