software/erp5: add control for PYTHONHASHSEED

See https://docs.python.org/3/using/cmdline.html#envvar-PYTHONHASHSEED

Setting a value will set the environment variable for all zope processes
and for the test runner.

Default behavior is:
  - for zope: do not set the variable, so default python behavior will
  be used ( equivalent to setting `0` on python2 and `random` on python3)
  - for test runner: generate a random value for each execution and
  print it, to make it easy to re-run a failing test with the same seed.

This means that ERP5 tests will likely reveal problems where code
depends on python2 behavior of deterministic hashing. To keep previous
behavior (and hide these problems), it's possible to set
python-hash-seed to 0 in test suite parameters.