Add tempest to minimal projects list
nodepool/elements/cache-devstack/install.d/99-cache-testrepository-db
touches the tempest dir, so is required even for a minimal build with
nodepool elements.  Add for clarity.
@@ -78,6 +78,7 @@ fi
## - project: openstack-infra/project-config
## - project: openstack-infra/system-config
## - project: openstack-dev/devstack
+## - project: openstack/tempest
# export DIB_CUSTOM_PROJECTS_LIST_URL='file:///tmp/custom_projects.yaml'
