specify location of image when using existing resource group

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#### Description

When using an existing resource group, packer will create the builder VM in the region of the resource group (the [`location` option can only be used when creating a new resource group](https://www.packer.io/plugins/builders/azure/arm#resource-group-usage)). However, this may not the region where the image is actually needed.

It would be useful, if the destination region for the VM could be specified separately from the resource group.

#### Use Case(s)

In corporate environments, a user may not have permissions to create new resource groups.

The current workaround is to
- create a separate vnet in the region of the resource group
- build the image there
- then replicate the image to the destination region

#### Potential configuration

```
```

#### Potential References

The payload names no source files or tests; start at the Azure ARM builder's handling of existing resource groups and the linked resource-group usage documentation. Done means an existing resource group can use a separately specified VM region, without requiring a new resource group, and the behavior is documented.

Written by the indexing model from the issue text.