[OVN] Re-enables neutron-ovn-tempest-slow job (voting)
[OVN] Re-enables neutron-ovn-tempest-slow job (voting)

This job was moved to the experimental gate due to gate failures with
the multinode setup. Those failures are now fixed.

Change-Id: I4a5cb5d7085043b1a4e0e31bb793229a32290010
Signed-off-by: Lucas Alvares Gomes <ef89b923b1145c9ec2fd96eee513279016a7a963@gmail.com>

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron