Delete *.pyc files before running tox tests

When jumping between different review branches .pyc files of newly
introduced .py files are getting left behind. When these are related to
DB migrations all tests start to fail with: "AssertionError: There is no
script for XY version". This is pretty annoying, so why not remove all
*.pyc files before running any tests? This commit implements that.

Change-Id: Iac223fb156af3729b51fc2b11323a7cc30f9c07d

tox.ini

@@ -20,9 +20,13 @@ deps = -r{toxinidir}/test-requirements.txt
 # to ncpu, to specify something else use
 # the concurrency=<n> option.
 # call ie: 'tox -epy27 -- --concurrency=4'
-commands = ostestr {posargs}
+commands =
+  find . -type f -name "*.pyc" -delete
+  ostestr {posargs}
 
-whitelist_externals = bash
+whitelist_externals =
+  bash
+  find
 passenv = *_proxy *_PROXY
 
 [testenv:api-ref]