520a979 Add singleton system_resolver

Authored and Committed by cqi 5 years ago
    Add singleton system_resolver
    
    system_resolver is created based on loaded configuration, which could
    avoid calls like `GenericResolver.create(conf)` repeatedly in the code.
    
    However, if some cases need to create a specific resolver explicitly,
    `GenericResolver.create` could be called with addition argument, for
    example db or mbs is passed to argument backend in tests.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>
    
        
file modified
+17 -14