Skip docs job for ironic-lib in stable/mitaka
Skip docs job for ironic-lib in stable/mitaka

docs appeared in ironic-lib only in Newton release, thus this job always
fails on stable/mitaka currently.

Change-Id: Ifd88ccd2afd7ee82108ded48bbe301e23148b63f

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