The vmware-nsx project is the only one using safe_reference [1] from the
common db mixin. We plan to remove safe_reference from the mixin, so
this patch switches vmware-nsx over to use weakref.proxy directly.
[1] http://codesearch.openstack.org/?q=%5C.safe_reference
Change-Id: I921c9265a646c3c291813d4488ad0fe0143038ca