Bugfix: Possible use of unknown key when str(beat.core.Host) is used

beat/core/dock.py

@@ -93,7 +93,7 @@ class Host(object):
 
 
   def __str__(self):
-    return self.kwargs['base_url']
+    return self.kwargs.get('base_url', 'Host')
 
 
   def env2docker(self, key):