1100892 - check if filename exists before printing

If a user uploads metadata like a repo category via pulp-admin and then lists
pending uploads, the list command will incorrectly look for an upload filename
on the tracker and then error when none is found.

Instead, only print the filename if it exists, and print "Metadata Upload"
otherwise.
Merge pull request #996 from beav/metadata-list

1100892 - check if filename exists before printing