function app: AZ CLI displaying misleading error message when creating Y1 app

## Describe the bug

**Command Name**
`az functionapp plan create`

**Errors:**
```
Invalid sku(pricing tier), please refer to command help for valid values
```

## To Reproduce:

- `az functionapp plan create --name foo --sku Y1 --is-linux false -g bar`
- Consumption plan is not created. Instead misleading error message is shown
- The SKU exists in Azure but Y1 is not hardcoded at AZ CLI source code

## Expected Behavior

- Requested resource should be created
- or AZ CLI should give guidance that Y1 plans must be created with `az functionapp create --consumption-plan-location ...`

## Environment Summary
```
macOS-10.15.5-x86_64-i386-64bit
Python 3.8.2
Installer: HOMEBREW

azure-cli 2.5.1 *

```
## Additional Context