DS-2119. EZID IdentifierProvider Metadata Mapping.

Added DataCite XML based metadata mapping option for EZIDIdentifierProvider module

https://jira.duraspace.org/browse/DS-2119
Including an empty Identifier property will solve that problem.

We at UMD use datacite xml to create our EZID records. Our datacite xml includes the identifier property as follows:   <identifier identifierType="DOI" />

See: http://ezid.cdlib.org/doc/apidoc.html (highlight: http://pages.citebite.com/e3v8n8l3k8etx)