[Experiment][Compute] New-AzVm shoulb be idempotent.

[sergey-shandar]

Currently, New-AzVm relays on a name to create or use a resource.

• To be idempotent, the operation should relay on a resource state. For example, the operation should create a new resource if a resource doesn't exist,
• otherwise the operation should get, validate, and modify (if necessary) the existing resource.
• Another issue is inferred location.

[cormacpayne]

Closing in favor of templates