Work around PYTHONHASHSEED-related problems

The JJB unit tests no longer pass, due to an upgraded version of
tox which sets PYTHONHASHSEED to random values.  Since the ordering
problem introduced is a difficult one to smash, this just sets
PYTHONHASHSEED to 0.  This also fixes the syntax of the
"[testenv]setenv" configuration setting.

Change-Id: I61fdbdf1a5099bcbacaeba59ff5001bb23df8cfd
Related-Bug: #1348818