Change of the name of the BatchServiceClient class parameter to the one consistent with the documentation. From base_url -> (#268)

`batch_url`.
Wrong name caused the error at run time:
```
TypeError: __init__() got an unexpected keyword argument 'base_url'
```

Author: k0staa

Python/Batch/sample1_helloworld.py

@@ -105,7 +105,7 @@ def execute_sample(global_config, sample_config):
 
     batch_client = batch.BatchServiceClient(
         credentials,
-        base_url=batch_service_url)
+        batch_url=batch_service_url)
 
     # Retry 5 times -- default is 3
     batch_client.config.retry_policy.retries = 5