networking-midonet: Make centos-7 job non-voting
networking-midonet: Make centos-7 job non-voting

Because these days it's failing too frequently due to
a problem which is seemingly unrelated to the code in this repo.
(maybe an infra issue)

Related-Bug: #1728602
Change-Id: I475a0cb7760b245d20776d54fd16ed9905af8f01

openstack-infra/project-config,openstack-infra/project-config