aks: add --enable-pod-identity-with-kubenet flag

[bcho]

This pull request implemented support for enabling AAD Pod Identity addon in AKS cluster using Kubnet network plugin. With the 2021-02-01 API version, we can:

• create cluster, with Kubnet network plugin + enable AAD Pod Identity addon:

$ az aks create -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --enable-pod-identity --enable-pod-identity-with-kubenet

If we don't set this flag, we will raise following error:

$ az aks create -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --enable-pod-identity 
The behavior of this command has been altered by the following extension: aks-preview
--enable-pod-identity-with-kubenet is required for enabling pod identity addon when using Kubenet network plugin

• update cluster that using Kubnet network plugin to enable AAD Pod Identity addon:

$ az aks update -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --enable-pod-identity --enable-pod-identity-with-kubenet
# no change for disabling addon
$ az aks update -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --disable-pod-identity

For cluster using Azure CNI network plugin, this flag is no-op. User can still toggle this addon with following commands:

$ az aks create -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --enable-pod-identity --network-plugin azure
$ az aks create -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --disable-pod-identity
$ az aks create -g $MY_RESOURCE_GROUP -n $MY_CLUSTER --enable-pod-identity

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[bcho]

@fengzhou-msft could you help review this PR? Thanks!