Ansible Galaxy content importer is using format strings [1] which
are supported since Python 3.6. Our Zuul CI job for OpenStack Queens
uses Ubuntu 16.04 LTS (Xenial Xerus) as its base image which has
Python 3.5 only.

Ref.:
[1] https://github.com/ansible/galaxy-importer/blob/b7140d6b3b5e172553788ae9aaa7526f8ff49ea2/galaxy_importer/main.py#L117

Change-Id: I5d3b2f71937a0e4ab9a8d49df10744f7d95a7de2