Switch gate-tempest-devstack-vm-neutron-large-ops to a voting job

There were some changes in Nova to stabilize large-ops jobs under
neutron. Let's please switch this to voting now to prevent regression

Compare the following logstash queries to see the difference since the
Nova changes went in last night (https://review.openstack.org/#/c/56075/)

"Finished: FAILURE" AND build_name:"gate-tempest-devstack-vm-neutron-large-ops"
"Finished: SUCCESS" AND build_name:"gate-tempest-devstack-vm-neutron-large-ops"

Change-Id: Iffcd7887a25ee74f57261917f0cfb4c0cd05bfc9
Merge "Switch gate-tempest-devstack-vm-neutron-large-ops to a voting job"