az appservice plan create - cannot create zone redundant plan for WS1 worker

**Related command**
```
az appservice plan create -g myrg -n myplan --sku WS2 --zone-redundant --number-of-workers 3 --location northeurope
```

**Describe the bug**
The command does not allow a zone redundant WS1, WS2 or WS3 plan to be defined. It is a valid Azure option to use zone redundancy with these plan types. The error message is:

```
Zone redundancy cannot be enabled for sku WS1
```

**To Reproduce**
Run the the above command after logging into Azure and setting you preferred default subscription and defining a resource group named ```myrg```.

**Expected behavior**
Allow WS1, WS2 and WS3 plans to be deployed with zone redundancy.