`az vmss create` has incorrect authorization for gallery images

## Describe the bug

**Command Name**
`az vmss create`

**Errors:**
```
ValidationError: (AuthorizationFailed) The client {} with object id {} does not have authorization to perform action 'Microsoft.Compute/galleries/images/read' over scope '/subscriptions/${sub_id}/resourceGroups/${rg}/providers/Microsoft.Compute/galleries/${gallery_name}/images/${image_name}' or the scope is invalid. If access was recently granted, please refresh your credentials.
```

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

```sh
az vmss create \
--name {} \
--resource-group {} \
--location {} \
--computer-name-prefix {} \
--vm-sku {} \
--instance-count {} \
--image /subscriptions/${sub_id}/resourceGroups/${rg}/providers/Microsoft.Compute/galleries/${gallery_name}/images/${image_name}/versions/${version_name} \
--priority {} \
--disable-overprovision \
--eviction-policy {} \
--scale-in-policy {} \
--zones {} \
--single-placement-group {} \
--platform-fault-domain-count {} \
--accelerated-networking {} \
--vnet-name {} \
--subnet {} \
--nsg {} \
--public-ip-address {} \
--authentication-type {} \
--admin-username {} \
--ssh-key-values {}
```

## Expected Behavior

Create the vmss successfully without authorization issue.

## Environment Summary
```
Linux-4.4.0-131-generic-x86_64-with-debian-stretch-sid
Python 3.6.10
Installer: DEB

azure-cli 2.18.0
```
## Additional Context

I have the read permission to one version of gallery image (resource id: `/subscriptions/${sub_id}/resourceGroups/${rg}/providers/Microsoft.Compute/galleries/${gallery_name}/images/${image_name}/versions/${version_name}`), and can create vmss successfully using this image id in ARM template, but cannot create vmss using the same image id in azure cli, which will throw authorization error.

The root cause is that azure cli also checked image permission (resource id: `/subscriptions/${sub_id}/resourceGroups/${rg}/providers/Microsoft.Compute/galleries/${gallery_name}/images/${image_name}`), instead of the given version.
https://github.com/Azure/azure-cli/blob/da4239a3b5ae9e2cc74785522bbb11f27b26eaf4/src/azure-cli/azure/cli/command_modules/vm/_validators.py#L471-L476

The `image_info` in above code shouldn't be got when user only has read permission to the given image version.