* Use byte strings for HTTP body
* Remove debug print()
* tox.ini: add cornice-py3 test environment
Note: It looks like it doesn't work to inherit commands from cornice
into cornice-py3 because cornice commands use the substitution string
{posargs}.
Change-Id: Iaed39babc7ee9877d443c47c2f4b21b2e98b5181