956839 - nodes only download file if does not exist on the child already.

https://bugzilla.redhat.com/show_bug.cgi?id=956839

SHORT TERM: For now, just checking to see if the file at storage_path exists.  This works for RPM repositories which is curently our main use case for large repositories since the storage path already includes the checksum for rpm units.  Having the node distributor calculate the checksum more than doubled the publishing time. 

LONG TERM: Discussed with jay, and it makes more sense to calculate and inject the checksum as a 1st class concept in the platform (perhaps in the conduit) so we only eat the cost once and only for newly imported units.  Also, makes sense that the platform store the checksum as a 1st class attribute on content units to support other integrity checking within the platform.