Add concurrency support in new integration tests

Add support of parallel running tests.
Testr provide ability to setting number of processes for running tests.
By default concurrency = <num_of_cores_in_cpu>

partially implements bp: scenario-integration-tests

Change-Id: Iebe570382e463b485fbcb57608c3d6d82690e098

etc/scenario/simple-testcase.yaml

@@ -1,3 +1,5 @@
+concurrency: 1
+
 credentials:
     os_username: admin
     os_password: nova

sahara/tests/scenario/runner.py

@@ -92,8 +92,12 @@ def main():
                                 path=test_dir_path)
 
     # run tests
+    concurrency = config.get('concurrency')
     os.environ['DISCOVER_DIRECTORY'] = test_dir_path
-    return_code = os.system('bash tools/pretty_tox.sh')
+    command = 'bash tools/pretty_tox.sh'
+    if concurrency:
+        command = command + ' -- --concurrency %d' % concurrency
+    return_code = os.system(command)
     sys.exit(return_code)