Make the CentOS 7 scenario gate non-voting
Make the CentOS 7 scenario gate non-voting

We have seen a lot of instability in the octavia-v2-dsvm-scenario-centos-7
gate[1] over the last few months. In the weekly IRC meeting we decided to
revert this gate back to non-voting until the CentOS issues are resolved.

We have seen:
1. Package repository issues.
2. Mirror issues.
3. CentOS 7.6 dropping packages.

[1] http://zuul.openstack.org/builds?job_name= \
        octavia-v2-dsvm-scenario-centos-7

Change-Id: I0d9ffc639b9decc9d13038d3033c61f6ba261ffe

openstack/octavia,openstack/octavia,openstack/octavia