Add __contains__ to ModelBase to fully behave like a dict

Oslo Db ModelBase implements __getitem__, __setitem__ but does not
implement __contains__. It behaves misleading on "in" operations,
silenlty returns False even if such key exists. Add this missing
method.

Change-Id: I1ddea1312424da8c372bb599371481ab76c19a3a
Closes-Bug: #1333337
Merge "Add __contains__ to ModelBase to fully behave like a dict"