Merge branch 'increase-retries' into 'master'

Retry twice the system, API and unknown failures

See merge request gitlab-org/gitlab-ce!27467

.gitlab/ci/global.gitlab-ci.yml

 .dedicated-runner:
   retry:
-    max: 1 # This is confusing but this means "2 runs at max".
+    max: 2 # This is confusing but this means "3 runs at max".
     when:
       - unknown_failure
       - api_failure