Fix and simplify reboot stress test. Add new env stuff.

Added stress environment var for max_instances.
Added tempest var for log_level of rest_client.

Change-Id: Ia92536a547cdab4d2496bd9ba9067be3595b79cd

xxxxx

Change-Id: I8c2f499a41f74c2fe6fb08c80ab6fc31f6b93426
diff --git a/stress/tests/hard_reboots.py b/stress/tests/hard_reboots.py
index f38ef6f..1b5928e 100644
--- a/stress/tests/hard_reboots.py
+++ b/stress/tests/hard_reboots.py
@@ -34,4 +34,4 @@
                sleep_time=500,  # in milliseconds

                seed=int(time.time()),

                test_name="hard reboots",

-               max_vms=32)

+               )