Add command deleting *.pyc before executing ostestr

This commit adds a command for deleting *.pyc before executing ostestr
with tox. This patch would help that removing unnecessary *.pyc files
when we rename, move or remove *.py ones.

Change-Id: I5fcce2e50cb7af9206175f340dad151e3eee6a9c

tox.ini

@@ -7,9 +7,11 @@ skipsdist = True
 usedevelop = True
 install_command = pip install -U --force-reinstall {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+whitelist_externals = find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
+    find . -type f -name "*.pyc" -delete
     ostestr {posargs}
 
 [testenv:pep8]