Right now InstanceMapping exposes the cell_mappings.id db field which is
just an implementation detail of the db relationship and should not be
exposed in the object. It is also useless as a way to look up the actual
CellMapping through that object interface. And InstanceMapping is only
looked up in order to find a CellMapping.
So the CellMapping should be joined during the db query and loaded on
the InstanceMapping object by default.
Change-Id: Ia8691b76bba310327bfe0995964525409794d1af