diff --git a/defaults/main.yml b/defaults/main.yml
index f59f78f4..be207385 100644
--- a/defaults/main.yml
+++ b/defaults/main.yml
@@ -228,5 +228,11 @@ horizon_pip_packages:
   - python-memcached
   - python-keystoneclient
 
+# This variable is used by the repo_build process to determine
+# which host group to check for members of before building the
+# pip packages required by this role. The value is picked up
+# by the py_pkgs lookup.
+horizon_role_project_group: horizon_all
+
 # Set arbitrary horizon configuration options
 horizon_config_overrides: {}