Remove build files before run tox doc builder
Remove build files before run tox doc builder

This may cause build failed if the old build file still existed.
This patch is to remove build files before run tox releasenotes
and docs builder.

Change-Id: Ia36abd03727ff3d71c6c24a71dcd4b5f7dbf9f77

StackStorm/mistral,StackStorm/mistral,openstack/mistral,openstack/mistral