Use existing (already created) storage account in monitoring

[Reintjuu]

On https://github.com/kumarvna/terraform-azurerm-mssql-db/blob/master/main.tf#L416 only the given var.storage_account_id is used. I think it should be configurable to be reusable from the already created Storage Account (https://github.com/kumarvna/terraform-azurerm-mssql-db/blob/master/main.tf#L40).

Another option would be to allow an external Storage Account to be provided to the module. Just like the parameterized/optional resource group creation.