Merge branch 'support-gitlab-qa-post-status-to-upstream-commit' into 'master'

Allow to override the gitlab-qa ref for the downstream trigger

See merge request gitlab-org/gitlab-ce!22521

scripts/trigger-build

@@ -107,7 +107,8 @@ module Trigger
       {
         'GITLAB_VERSION' => ENV['CI_COMMIT_SHA'],
         'ALTERNATIVE_SOURCES' => 'true',
-        'ee' => Trigger.ee? ? 'true' : 'false'
+        'ee' => Trigger.ee? ? 'true' : 'false',
+        'QA_BRANCH' => ENV['QA_BRANCH'] || 'master'
       }
     end
   end