TrivialFix flake8 and docs build in tox.ini

1 "build" is a generated doc file directory we need to exclude.
2 to ensure the old files are not left over when running these
  document builds.

Change-Id: Ieb8fb314375b5a8ac3f38b1944d514937274c8c6

tox.ini

@@ -46,7 +46,10 @@ commands =
   python setup.py build_sphinx
 
 [testenv:releasenotes]
-commands = sphinx-build -a -E -W -d releasenotes/build/doctrees \
+whitelist_externals = rm
+commands =
+    rm -rf releasenotes/build
+    sphinx-build -a -E -W -d releasenotes/build/doctrees \
          -b html releasenotes/source releasenotes/build/html
 
 [testenv:functional]
@@ -78,4 +81,4 @@ builtins = _
 # [H203] Use assertIs(Not)None to check for None.
 # [H904] Delay string interpolations at logging calls.
 enable-extensions = H106,H203,H904
-exclude = .venv,.tox,dist,doc,*egg
+exclude = .venv,.tox,dist,doc,*egg,build