Add "clean" tox target

This uses the -c parameter to process-templates.py to clean the
generated templates in the local repo.

Change-Id: Ib105f60a1e54a6d6372d00bb2db6a9b378b2176e

tox.ini

@@ -28,6 +28,9 @@ commands =
 [testenv:templates]
 commands = python ./tools/process-templates.py
 
+[testenv:clean]
+commands = python ./tools/process-templates.py -c
+
 [testenv:releasenotes]
 commands = bash -c tools/releasenotes_tox.sh