For compliance with the Project Testing Interface [1]
as described in [2]

[1]
https://governance.openstack.org/tc/reference/project-testing-interface.html
[2]
http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html

doc8 command is dropped from docs tox envs.
So this affect nothing and run in PEP8.

Related-Bug: #1765348

Depends-On: Icc7fe3a8f9716281de88825e9d5b2fd84de3d00a
Change-Id: Idf9a16111479ccc64004eac9508da575822a3df5