Ignore python warnings in the fullstack job

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
specific directories.

Change-Id: I080381cb8a88a27130ac21c0e27fc915e38bdedd
(cherry picked from commit 14955f93f6)

tox.ini

@@ -91,6 +91,9 @@ setenv = {[testenv]setenv}
          # workaround for DB teardown lock contention (bug/1541742)
          OS_TEST_TIMEOUT={env:OS_TEST_TIMEOUT:600}
          OS_TEST_PATH=./neutron/tests/fullstack
+# Because of issue with stestr and Python3, we need to avoid too much output
+# to be produced during tests, so we will ignore python warnings here
+         PYTHONWARNINGS=ignore
 deps =
   {[testenv:functional]deps}
 commands =