Refactor PungiConfig
    
    Raw pungi config and non raw pungi config are deteremined twice, and in
    the second time, raw pungi config is determined by check if passed in
    value has dict type. While I'm reading through the code and try to
    understand the logic, I found I'm confused easily by that.
    
    Instead, RawPungiConfig class is introduced to represent the raw config,
    and both RawPungiConfig and original PungiConfig implement method from
    BasePungiConfig to write options to config file, which could be simplify
    code a bit.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>