fix session connection

previous session flushing fix caused conflict between database_connection
and [database]connection values. correct the behaviour in this fix.

also, remove empty return statement at end of record_metering_data function.

Change-Id: I3b2525cec05f5ee2ee8277bfbdca52b9a768974b

ceilometer/storage/__init__.py

@@ -59,7 +59,8 @@ def register_opts(conf):
 def get_engine(conf):
     """Load the configured engine and return an instance."""
     if conf.database_connection:
-        conf.database.connection = conf.database_connection
+        conf.set_override('connection', conf.database_connection,
+                          group='database')
     engine_name = urlparse.urlparse(conf.database.connection).scheme
     LOG.debug('looking for %r driver in %r',
               engine_name, STORAGE_ENGINE_NAMESPACE)

ceilometer/storage/impl_mongodb.py

@@ -322,7 +322,6 @@ class Connection(base.Connection):
         # a new key '_id').
         record = copy.copy(data)
         self.db.meter.insert(record)
-        return
 
     def get_users(self, source=None):
         """Return an iterable of user id strings.

ceilometer/storage/impl_sqlalchemy.py

@@ -212,8 +212,6 @@ class Connection(base.Connection):
             meter.message_id = data['message_id']
             session.flush()
 
-        return
-
     def get_users(self, source=None):
         """Return an iterable of user id strings.