Add doc8 to pep8 environment

OpenStack CI does not run the docs environment, it's just for developer
convenience. Add the doc8 to the general linter environment so that it's
run during automatic testing. The general linter environment is pep8.

Disable testing of history.rst since generated ChangeLog file gives an
error:
"doc/source/user/history.rst:150: D000 Inline emphasis start-string
without end-string."

Readd sphinx to test environment so that doc8 passes tests.

Change-Id: Ia81eac686ff3a82273b1d306596c4731c86c36e6

setup.cfg

@@ -80,6 +80,7 @@ test =
   testtools>=2.2.0 # MIT
   testscenarios>=0.4 # Apache-2.0/BSD
   doc8>=0.6.0 # Apache-2.0
+  sphinx>=1.6.2 # BSD
 
 [nosetests]
 cover-erase = true

tox.ini

@@ -30,7 +30,9 @@ deps = {[testenv]deps}
 commands = {toxinidir}/tools/update_states.sh
 
 [testenv:pep8]
-commands = flake8 {posargs}
+commands =
+    flake8 {posargs}
+    doc8 doc/source
 
 [testenv:pylint]
 deps = {[testenv]deps}
@@ -54,6 +56,13 @@ import_exceptions = six.moves
     taskflow.test.mock
     unittest.mock
 
+[doc8]
+# Settings for doc8:
+# Ignore doc/source/user/history.rst, it includes generated ChangeLog
+# file that fails with "D000 Inline emphasis start-string without
+# end-string."
+ignore-path = doc/*/target,doc/*/build*
+
 [testenv:py27]
 deps = .[test,workers,zookeeper,database,redis,eventlet]
       -r{toxinidir}/doc/requirements.txt