Parameter --zone is different from the parameter used on other resources to accomplish the same objective

The parameter (--zone) to specify the list of availability zones into which provision the resource for a public ip (az network public-ip create) is different from the parameter with the same purpose for other resources such as application-gateway and vmss, where for both the parameter is --zone**s**.

To keep a standard, should better if we can have this parameter exactly like it's used in other resources (--zones instead of --zone)

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* ID: c12e07f8-d82f-a9ac-895a-b6d980c8c4c2
* Version Independent ID: 43924bb5-20bd-6a0e-9cdc-90b1db46e599
* Content: [az network public-ip](https://docs.microsoft.com/en-us/cli/azure/network/public-ip?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/network/public-ip.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/network/public-ip.yml)
* Service: **virtual-network**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**