Remove doc/build during tox -e docs

This removes the previously built docs so that
we get a fresh build each time to pick up the
latest changes.

Change-Id: I3c5da78f2aee85810eabe7c71cd89caa0d65e541

tox.ini

@@ -5,6 +5,8 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
+whitelist_externals =
+  rm
 install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
@@ -38,7 +40,9 @@ commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
 basepython = python3
-commands = python setup.py build_sphinx
+commands =
+  rm -rf doc/build
+  python setup.py build_sphinx
 
 [testenv:releasenotes]
 basepython = python3