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/tools/generate-tempest-plugins-list.py b/tools/generate-tempest-plugins-list.py
index bbb9019..4eb78fb 100644
--- a/tools/generate-tempest-plugins-list.py
+++ b/tools/generate-tempest-plugins-list.py
@@ -63,12 +63,13 @@
     except HTTPError as err:
         if err.code == 404:
             return False
-    p = re.compile('^tempest\.test_plugins', re.M)
+    p = re.compile(r'^tempest\.test_plugins', re.M)
     if p.findall(r.read().decode('utf-8')):
         return True
     else:
         False
 
+
 r = urllib.urlopen(url)
 # Gerrit prepends 4 garbage octets to the JSON, in order to counter
 # cross-site scripting attacks.  Therefore we must discard it so the