Configure branch on which tripleo jobs should be run

Those jobs are run in the periodic queue and it seems that they are
using neutron from the last stable release instead of master.
So this patch sets explicitly master branch to be used.

Change-Id: Ibef0c6f33f68394797d623ced709e079fe6e22e0
Closes-Bug: #1923201

zuul.d/tripleo.yaml

@@ -2,6 +2,7 @@
     name: neutron-centos-8-tripleo-standalone
     parent: tripleo-ci-base-standalone-centos-8
     voting: false
+    branches: ^master$
     vars:
       featureset: '052'
       featureset_override:
@@ -36,6 +37,7 @@
 - job:
     name: neutron-ovn-tripleo-ci-centos-8-containers-multinode
     parent: tripleo-ci-centos-8-containers-multinode
+    branches: ^master$
     vars:
       nodes: 1ctlr
       featureset: '010'