Handle default type name as str in param sources

With this commit our parameter sources handle the default type name as
string by default and implement a fallback to treat it as a structured
object (as in earlier versions of Rally)

Relates #7

eventdata/parameter_sources/elasticlogs_bulk_source.py

@@ -68,7 +68,8 @@ def __init__(self, track, params, **kwargs):
             logger.debug("[bulk] Index pattern specified in parameters ({}) will be used".format(params['index']))
 
         if 'type' not in params.keys():
-            self._params['type'] = self._indices[0].types[0].name
+            t = self._indices[0].types[0]
+            self._params['type'] = t if isinstance(t, str) else t.name
 
     def partition(self, partition_index, total_partitions):
         return self

eventdata/parameter_sources/sample_based_bulk_source.py

@@ -100,7 +100,8 @@ def __init__(self, track, params, **kwargs):
             logger.debug("[bulk] Index pattern specified in parameters ({}) will be used".format(params['index']))
 
         if 'type' not in params.keys():
-            self._params['type'] = self._indices[0].types[0].name
+            t = self._indices[0].types[0]
+            self._params['type'] = t if isinstance(t, str) else t.name
 
         if 'timestamp_field' not in params.keys():
             self._params['timestamp_field'] = []