Replace deprecated perl-style regex

... according to the following warning posted by zuul

```
All regular expressions must conform to RE2 syntax, but an expression
using the deprecated Perl-style syntax has been detected. Adjust
the configuration to conform to RE2 syntax.
```

Change-Id: I770244a37b2b8e05d38f892aa5519784547defd2

.zuul.yaml

@@ -29,7 +29,9 @@
       - openstack/python-zaqarclient
       - openstack/python-zunclient
       - openstack/requirements
-    branches: ^(?!stable)
+    branches:
+      regex: ^stable/.*$
+      negate: true
 
 - project-template:
     name: openstackclient-plugin-jobs