make metadata indexing optional

while indexing the rally results to ES make the metadata indexing
optional.

now we can remove the metadata field in browbeat-config.yml and
disable the metadata indexing.

Change-Id: I68fcf4153f79bc321d920203525fea228fa0e996

browbeat/elastic.py

@@ -73,9 +73,7 @@ class Elastic(object):
         return json_data
 
     def combine_metadata(self, result):
-        if (self.config['elasticsearch']['metadata_files'] is not None and
-                len(self.config['elasticsearch']['metadata_files']) > 0):
-            meta = self.config['elasticsearch']['metadata_files']
+        meta = self.config['elasticsearch'].get('metadata_files', []) or []
         for _meta in meta:
             try:
                 with open(_meta['file']) as jdata:

browbeat/schema/browbeat.yml

@@ -62,7 +62,7 @@ mapping:
         required: True
       metadata_files:
         type: seq
-        required: True
+        required: False
         sequence:
           - type: map
             mapping:

browbeat/tools.py

@@ -70,7 +70,7 @@ class Tools(object):
         workloads.run_workload(copy.deepcopy(workload), run_iteration)
 
     def check_metadata(self):
-        meta = self.config['elasticsearch']['metadata_files']
+        meta = self.config['elasticsearch'].get('metadata_files', []) or []
         for _meta in meta:
             if not os.path.isfile(_meta['file']):
                 self.logger.info(