Validate VM creation using --nics

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az vm create`

**Errors:**
```
When specifying an existing NIC, do not specify NSG, public IP, ASGs, VNet or subnet.
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az vm create -g {} -l {} --name {} --image {} --size {} --nics {} --public-ip-address {} --public-ip-sku {} --subnet {} --nsg {} --nsg-rule {} --admin-username {} --authentication-type {} --ssh-key-values {} {} --validate`

## Expected Behavior

Validation successful since the nic doesn't exist yet

## Environment Summary
```
Linux-5.4.0-54-generic-x86_64-with-debian-buster-sid
Python 3.6.10
Installer: DEB

azure-cli 2.15.1

Extensions:
aks-preview 0.4.66
azure-firewall 0.7.0

```
## Additional Context