Skip security role gate for branches <= Mitaka

Somewhere in the consolidation of the OSA zuul job cleanup, CentOS 7
jobs were re-enabled for the openstack-ansible-security role in
Liberty/Mitaka. The role didn't support CentOS 7 in those branches
and this patch should disable those tests.

Change-Id: I85d5f85a6b78d945b635c7ec42f741bb77e2fb64

zuul/layout.yaml

@@ -3167,6 +3167,11 @@ jobs:
   - name: ^gate-openstack-ansible-openstack-ansible-aio-ubuntu-xenial$
     branch: ^(?!(liberty|stable/(mitaka|newton))).*$
 
+  # Skip the functional tests for CentOS 7 in the openstack-ansible-security
+  # role if the branch is Mitaka or earlier.
+  - name: ^gate-openstack-ansible-security-ansible-func-centos-7$
+    branch: ^(?!(liberty|stable/mitaka)).*$
+
   # Skip the functional tests against the RHEL 7 STIG content on Newton branch
   # and earlier branches.
   - name: ^gate-openstack-ansible-security-ansible-func_rhel7.*$