Implements updating complete bw usage data

the DB query is not filtering by the mac address.
Hence, only one of them was being updated and
with mismatched bw usage info.

fixes bug 1021961

Change-Id: I792493a6e0817d62bbf6955df9cd5bc0e4af9dbb

nova/db/sqlalchemy/api.py

@@ -4371,6 +4371,7 @@ def bw_usage_update(context,
                               session=session, read_deleted="yes").\
                       filter_by(start_period=start_period).\
                       filter_by(uuid=uuid).\
+                      filter_by(mac=mac).\
                       first()
 
         if not bwusage: