pass uuid to Metadata.pm

to avoid "Need UUID or existing metadata file!" at Metadata.pm line 56
I don't think tests exercise this code, but if tests did exercise this code path, then we would have a test which depends on the availability and function of an external webservice.

By inspection, I don't think that Metadata->create works unless either a non-empty file or a uuid are passed to it.  Since the file is empty, the code in _retrieve_metadata_file() falls through to the uuid case and the metadata_file property is set again.

@thepler does this work if you remove line 206 (i.e., if you don't pass in $tmp_xml)?