Run openstackid docs jobs on bionic
Run openstackid docs jobs on bionic

The openstackid project is being updated to require a version of PHP
only available on Ubuntu Bionic and later, so to simplify builds of
its docs jobs  run them on ubuntu-bionic nodes where this bindep
requirement is already satisfied.

Change-Id: I94f56c10633c30f2487c990d6a1e6da0bd6581ba

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