listKeys not supported in variable

If I use `listKeys()` in a variable, I get the error:

> The template function 'listKeys' is not expected at this location

for example:

``` javascript
{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
"variables": {
"storageAccountKey": "[listKeys(resourceId('Microsoft.Storage/storageAccounts/mystorageaccount)),'2015-06-15').key1]"
}
}
```

Can support for listKeys inside a variable be added?

No repository files, tests, or entry points are named. Start by reviewing the ARM deployment-template example and the comment thread to determine the expected evaluation behavior; done means listKeys() is accepted inside a variable and the shown template resolves the storage account key successfully.

Written by the indexing model from the issue text.