Merge branch '9748-fix-boards-milestones-controller-spec' into 'master'

Add missing property percentage_complete to the milestone schema

Closes #9748

See merge request gitlab-org/gitlab-ee!9539

ee/spec/fixtures/api/schemas/entities/milestone.json

@@ -12,6 +12,7 @@
     "updated_at": { "type": "string" },
     "start_date": { "type": ["date", "null"] },
     "due_date": { "type": ["date", "null"] },
+    "percentage_complete": { "type": "integer" },
     "web_url": { "type": "string" }
   },
   "additionalProperties": false