Can you provide an interface to get `StorageConnectionString`(or something else) from `subscription` directly instead of creating ServiceManagementService at here? We need to decopule the `BlobService` and `ServiceManagementService`.

For example,

```
function getBlobServiceClient(options) {
   ....
   //get service client with env variables
  ...
  //get srevice client with susbscription
  var account = cli.category('account');
  account.getStorageConnectionString(subscriptionId, storageAccountName, function(error, connectionString) {
     if(error) throw error;
     return StorageUtil.getBlobService(connectionString);
  });
}
```