Update the alternate tox.ini files in test-playbooks/python to
trigger configuration we saw expose a bug in the siblings lib's
showconfig parser when a tox.requires value resulted in pip
activity. Update its minversion to the earliest tox which provides
the tox.requires configuration option, for clarity.
While we're there, switch to using tox_config_file instead of
overriding zuul_work_dir since that's now an option for this role
and was previously not exercised in the test job.
Change-Id: I521580da978903cd116c872447b1c028c470e63d
31a470245b