Batch command: avoid accessing potentially undefined context.principal

This might happen when the command is called directly in Python,
e.g. in installers and so on.

Pylint pylint-1.5.5-1.fc24.noarch caught this.

https://fedorahosted.org/freeipa/ticket/5838

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipalib/plugins/batch.py

@@ -107,7 +107,10 @@ class batch(Command):
 
                 result = api.Command[name](*a, **newkw)
                 self.info(
-                    '%s: batch: %s(%s): SUCCESS', context.principal, name, ', '.join(api.Command[name]._repr_iter(**params))
+                    '%s: batch: %s(%s): SUCCESS',
+                    getattr(context, 'principal', 'UNKNOWN'),
+                    name,
+                    ', '.join(api.Command[name]._repr_iter(**params))
                 )
                 result['error']=None
             except Exception as e: