Add custom azure-cni version back to staging template.
Add custom azure-cni version back to staging template.

The custom version was removed as the staging job was updated
to use aks-engine version 0.34.0 that defaults azure-cni to v1.0.18.
However, since that version of aks-engine does not properly handle
custom extensions scripts we have to revert to a known working
aks-engine version for the job, via:

https://github.com/kubernetes/test-infra/pull/12238

kubernetes-sigs/windows-testing,kubernetes-sigs/windows-testing