[services-ng] modify mongodb proxy package reference

   1. bump govendor submodule
   2. adapt steno to be logging handler
   3. adapt govendor to be package manager

   git shortlog 609686ec..6c5ff67e
   Chunjie (1):
       Add mongodb client golang driver, labix.org/v2/mgo.

Change-Id: I284f66af11254e9a3f4cbd7b754e4f93b61f1ded