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>