Fix stable branch selector in zuul layout

The branch selector intended to make sure that the -icehouse and
-havana devstack-gate branchless tests only run on master was also
catching the periodic-.*-havana jobs (and icehouse).  Add a negative
match for periodic to that regex.

Change-Id: I35289e80157cf7dac741250932742f75af7b7f55

modules/openstack_project/files/zuul/layout.yaml

@@ -428,7 +428,7 @@ jobs:
     parameter-function: set_node_options_default_trusty
   # tempest and d-g branchless tests need to only run on master
   # (needs to be early to not impact non-voting stanzas later)
-  - name: ^.*(-icehouse|-havana)$
+  - name: ^(?!periodic-).*(-icehouse|-havana)$
     branch: ^master.*$
   - name: gate-tempest-dsvm-full
     queue-name: integrated