Make parametrized tests xdist-compatible

[abalakh]

(robottelo issue)
Currently test runs with parametrized tests using xdist are failing with msgs like:

collecting 0 items / 3 errors
==================================== ERRORS ====================================
_____________________________ ERROR collecting gw1 _____________________________
Different tests were collected between gw0 and gw1. The difference is:

This is due to random data generated inside parametrize. We should probably use the same hash seed across different workers. More info here: pytest-dev/pytest#1075