rpmbuild: set Vendor metadata for builds
    
    This is configurable per Copr instance in main.ini (the
    main.rpm_vendor_copr_name option).
    
    The final Vendor format is e.g. "Fedora Copr - group @copr".  This
    should be useful in the future "sticky vendors" DNF's feature.
    
    Fixes: #1543
    Merges: #1548