Update neutron-l3-agent from 2:19.0.0-0ubuntu1~cloud0 to 2:19.1.0-0ubuntu2~cloud0 fixed the issue for me with cloud-archive:xena-proposed enabled.
I checked that the init script doesn't mention fwaas_driver.ini anymore, and neutron-l3-agent successfully restarts even if the fwaas_driver.ini file is not present.
Update neutron-l3-agent from 2:19.0. 0-0ubuntu1~ cloud0 to 2:19.1. 0-0ubuntu2~ cloud0 fixed the issue for me with cloud-archive: xena-proposed enabled.
I checked that the init script doesn't mention fwaas_driver.ini anymore, and neutron-l3-agent successfully restarts even if the fwaas_driver.ini file is not present.