Fixes errors after WSME updated to 0.5b5

WSME introduces new way of handling errors -
https://github.com/stackforge/wsme/commit/579772807ef9f1066bb28aa625f01539d2e95825

This changes how we should put errors into Response constructor.

0.5b5 release of WSME contains https://review.openstack.org/#/c/44659/
so we can drop our workaround https://review.openstack.org/#/c/44689/
by removing "six" from requirements.txt

Changes response status tests after delete of flavors b/c it now returns
correct 204.

Minor formatting in renderers.py

Change-Id: I8be35c41b583223f910f8e43be3a225df4d4d4ec