Remove exceptions.Duplicate
Remove exceptions.Duplicate

exceptions.Duplicate is largely unused (one fake and one use in
cloudpipe). We should replace those users with more specific
exceptions and then remove exceptions.Duplicate.

Closes-Bug: 1228953
Change-Id: I0e7b4bae5728600fb81511a05778419b3e5577df

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects