Staging1: reduce concurrent builds from 3 to 2

The staging1 build agents only have 4 CPUs and 4GB RAM, which is
insufficient to run the Artemis Spring server plus 3 concurrent
builds simultaneously. Reducing to 2 concurrent builds to prevent
resource exhaustion that was causing Docker daemon freezes.

Author: krusche

group_vars/artemis_staging1.yml

@@ -23,7 +23,7 @@ version_control:
 continuous_integration:
   localci:
     url: "{{ artemis_server_url }}"
-    concurrent_build_size: 3
+    concurrent_build_size: 2
     image_cleanup:
       enabled: true
       expiry_days: 7