Use xenial for translation jobs
Use xenial for translation jobs

Zanata-cli needs Java 8 and will fail with newer Java, Bionic uses Java 11
This fails Zanata up to version 4.6.2 (current release in March 2019).

Go back to Xenial for now.

Change-Id: I96eb4e36b9e73c7faaccc505ffee343b56cb36ea

openstack-infra/project-config,openstack-infra/project-config