Looking through the log you linked:
Invoking '/usr/local/lib/python2.7/dist-packages/ros_buildfarm-1.4.2_master-py2.7.egg/EGG-INFO/scripts/generate_doc_independent_job.py file:///root/ros_buildfarm_config/index.yaml independent-packages --dry-run'
Don’t the ros_buildfarm scripts require Python 3.x (see also ros_buildfarm/doc/environment.rst)?