Fix elasticsearch service on gitlab-ci

Overwrite the default command to pass discovery.type=single-node
Environment variable with "." can't be exported from bash.

See https://gitlab.com/gitlab-org/gitlab-runner/issues/2410

.gitlab-ci.yml

@@ -43,6 +43,7 @@ test:
     - redis:4.0
     - name: docker.elastic.co/elasticsearch/elasticsearch:7.3.2
       alias: elasticsearch
+      command: ["bin/elasticsearch", "-Ediscovery.type=single-node"]
   before_script:
     - pip install -r requirements-dev.txt
   script: