Add a modified transform to meet EZID requirements.

The EZID provider depends on the registrar to generate and return the
identifier, so it cannot supply one in the request. But the DataCite
XML schema requires an "identifier" element. A new transform is
offered, which generates an empty "identifier" element, satisfying
EZID.
Merge pull request #734 from mwoodiupui/DS-2119

[DS-2119] Add a modified transform to meet EZID requirements.