If there is too much output produced to the stdout/stderr during the
tests, stestr can hangs and job will then time out.
To avoid that in fullstack job, this patch is doing the same what
was already done for functional job as well - set PYTHONWARNINGS
variable to "ignore" to ignore all warnings and not send them to stdout.
All info really needed to debug tests is still in the tests' logs in
(cherry picked from commit