Update virtual-kubelet image to latest version

[mboersma]

Describe the request

Explain why AKS Engine needs it

Describe the solution you'd like

Describe alternatives you've considered

Additional context
See #2535 (comment)

[mboersma]

@feiskyer we are currently referencing virtual-kubelet:latest when we deploy the aci-connector addon. Is there anything to update?

(I tested this manually after #2535 merged and things seem to be working.)

% KUBECONFIG=_output/maboersm1/kubeconfig/kubeconfig.eastus.json k get no
NAME                                 STATUS   ROLES    AGE     VERSION
aci-connector                        Ready    agent    6m43s   v1.13.1-vk-v0.9.0-1-g7b92d1ee-dev
k8s-agentpool1-38830091-vmss000000   Ready    agent    7m59s   v1.18.0-alpha.1
k8s-agentpool1-38830091-vmss000001   Ready    agent    8m3s    v1.18.0-alpha.1
k8s-master-38830091-0                Ready    master   8m3s    v1.18.0-alpha.1

[feiskyer]

@mboersma cool, thanks a lot. I didn't realize it's already the latest.