Convert creation_time to string

The message to be serialized to JSON has a creation_time field
that is of datetime type. The json.dumps method is unable to
serialize this. Changing creation_time to timestamp based on
review comments and to keep it consistent with monasca-api.

Change-Id: Ibbff895e545788cb64b8733adcd6d060f11ef6c0

monasca_log_api/tests/test_service.py

@@ -313,9 +313,11 @@ class LogCreatorNewEnvelope(unittest.TestCase):
         tenant_id = 'a_tenant'
         none = None
         meta = {'tenantId': tenant_id, 'region': none}
+        timestamp = (datetime.datetime.utcnow() -
+                     datetime.datetime(1970, 1, 1)).total_seconds()
         expected_envelope = {
             'log': expected_log,
-            'creation_time': datetime.datetime.utcnow(),
+            'creation_time': timestamp,
             'meta': meta
         }
 

monasca_log_api/v1/common/service.py

@@ -199,9 +199,11 @@ class LogCreator(object):
         return log_object
 
     def new_log_envelope(self, log_object, tenant_id):
+        timestamp = (datetime.datetime.utcnow() -
+                     datetime.datetime(1970, 1, 1)).total_seconds()
         return {
             'log': log_object,
-            'creation_time': datetime.datetime.utcnow(),
+            'creation_time': timestamp,
             'meta': self._create_meta_info(tenant_id)
         }