[az aks nodepool] Add for membership of the managed identity to an AAD group.

## Problem
When a node pool is added, in an instance of AKS, the VMSS created comes with the managed identity disabled, this behavior makes it necessary to execute several other commands to discover the VMSS identifier, enable the managed identity, associate the new identity with KeyVault rules, SQL Server and other services that are used by the applications that will be executed.

## Suggestion:
add the parameter –-add-managed-identity-group

This parameter must be defined with the AAD group that will be linked to the VMSS System Identity

### How will it work:
At the end of command execution
`az aks nodepool add -g MyResourceGroup -n nodepool1 --cluster-name MyManagedCluster –-add-managed-identity-group ` the CLI should get the name of the VMSS that was created and run the command functions

`az vmss identity assign -g MyResourceGroup -n `
`az vmss identity show --name --resource-group MyResourceGroup` to read managed identity id.
finally you must execute the function of the command
` az ad group member add -–group --member-id `

### Summary
With a single parameter the user will be able to solve a complex problem, manage the permissions of the new nodepool, without running the risk of machines becoming available without the permissions to access their resources.