Ensure doc8 is run

When a doc8 issue has been detected, the CI job must fail.

Change-Id: Ic4a5867b5c38643231f5af97acb4d66878a2ae4d

tox.ini

@@ -153,6 +153,7 @@ commands =
 
 [testenv:linters]
 deps =
+    {[testenv:docs]deps}
     {[testenv:ansible]deps}
 commands =
     {[testenv:pep8]commands}
@@ -160,3 +161,4 @@ commands =
     {[testenv:ansible-lint]commands}
     {[testenv:ansible-syntax]commands}
     {[testenv:inventory]commands}
+    {[testenv:docs]commands}