Mark raw strings as such

This is raising warnings now and will be an error in future versions of
Python 3.

Change-Id: I8827cb36ef2ef85e8f245a2a181fb23ee75bfd16
diff --git a/tempest/cmd/init.py b/tempest/cmd/init.py
index 84c8631..d84f3a3 100644
--- a/tempest/cmd/init.py
+++ b/tempest/cmd/init.py
@@ -26,7 +26,7 @@
 
 LOG = logging.getLogger(__name__)
 
-STESTR_CONF = """[DEFAULT]
+STESTR_CONF = r"""[DEFAULT]
 test_path=%s
 top_dir=%s
 group_regex=([^\.]*\.)*