Add the publicIpPrefix prop during VmssUpdate

[karthikka4820]

When a customer tries to scale VMSS using Terraform, since the current definitions do not have publicIpPrefix property, the new VM will be assigned a random IP address which is outside the range of public Ip prefix. Customer has to resolve the issue by deleting the Vmss and recreating it. This PR tries to resolve the issue by adding the publicIpPrefix property in the JSON definition which is used to generate a request for VMSS update.

More details about the issue can be found here - Azure#10190

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