potential solution for https://review.openstack.org/#/c/444417 ?

gistfile1.txt

neutron_lbaasv2_service_providers: 
  - "{% if neutron_lbaasv2_service_provider | bool %}LOADBALANCERV2:Haproxy:neutron_lbaas.drivers.haproxy.plugin_driver.HaproxyOnHostPluginDriver{% endif %}"
  - "{% if neutron_lbaas_octavia | bool %}LOADBALANCERV2:Octavia:neutron_lbaas.drivers.octavia.driver.OctaviaDriver{% endif %}"

neutron_lbaasv2_service_provider: "{{ neutron_lbaasv2_service_providers | join(',') }}{% if neutron_lbaasv2_service_providers | length > 0 %}:default{% endif %}"

# If it rejects the empty string lines here, or the join includes empty lines causing extra commas, we might be able
# to work around it as such:
#   - add "{% elseif %}None" or something in each providers line (2, 3)
#   - use "neutron_lbaasv2_service_providers | reject('none')| join(',')" on line 5

https://gist.github.com/stevelle/1f7f82b7af787b0bc1097276d2a917a0 may be cleaner or work better also