Delete python bytecode before every test run

Because python creates pyc/pyo files during tox runs, certain
changes in the tree, like deletes of files, or switching
branches, can create spurious errors.

This patch removes these unrequired files when tox
starts execution, along with the __pycache__ folder for
py3* compilation.

Change-Id: Iea5f19ab8a4d56dedaa8c3a3950af041d3ff5b9f
Closes-Bug: #1368661

tox.ini

@@ -9,7 +9,10 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = ostestr --regex '{posargs}'
+commands = find . -type f -name "*.py[c|o]" -delete
+           find . -type d -name "__pycache__" -delete
+           ostestr --regex '{posargs}'
+whitelist_externals = find
 
 [tox:jenkins]
 sitepackages = True