Create discovery option for stress tests

Adds a decorator @stresstest which automatically sets the type
attr to "stress". It uses the testtools discover functionality to
automatically discover stress tests out of all tempest test.

It is possible to filter stress test with a given attribute type.
For instance to filter out only smoke tests that have a stress test
attribute.

blueprint: stress-tests
Change-Id: I8acf0b608cb500c2679a36a4a00ca4fa14668fad
diff --git a/tempest/config.py b/tempest/config.py
index 9b1a91e..c6a291e 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -515,7 +515,10 @@
                help='regexp for list of log files.'),
     cfg.StrOpt('log_check_interval',
                default=60,
-               help='time between log file error checks.')
+               help='time (in seconds) between log file error checks.'),
+    cfg.StrOpt('default_thread_number_per_action',
+               default=4,
+               help='The number of threads created while stress test.')
 ]