tox: Don't create '.pyc' files

Our tox.ini was trying to use find but failing because it was not
added to whitelist_externals. Rather than correct that, we can
remove it entirely by adjusting tox to no longer create '.pyc'
files, so we no longer have to delete them.

Nova previously did this [1] to improve performance, but then had
to revert it [2] because of a tox bug. That bug has since been
fixed [3], so we should be ok to do this.

[1] 0d6d1616b9a05f1c3e1e34cb235fee327196d423
[2] 233160644f4f0a17177fffe8a1e6db1341e61ad4
[3] https://review.openstack.org/#/c/582392/1/tox.ini@28

Change-Id: I11cbf81160ba2eeefcce19ed25930c39e17449b7

tox.ini

@@ -18,15 +18,16 @@ setenv =
   OS_STDOUT_CAPTURE=1
   OS_STDERR_CAPTURE=1
   OS_TEST_TIMEOUT=60
+  PYTHONDONTWRITEBYTECODE=1
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
   -egit+https://git.openstack.org/openstack/nova#egg=nova
   -rhttps://git.openstack.org/cgit/openstack/nova/plain/test-requirements.txt
-whitelist_externals = bash
+whitelist_externals =
+  bash
 commands =
-  find . -type f -name "*.pyc" -delete
 
 [testenv:py27]
 commands =