Comment 8 for bug 1554177

https://github.com/openstack/fuel-devops/blob/master/devops/models/environment.py#L158

this code actually causing this problem. libvirt works asynchronously and starting networks in parallel create race condition here. My suggestion is to add sleep there, or implement something like 'status' method for driver and wait for status, for libvirt that may be iptables-save | grep bridge or something like that