Refactor get_parent_image_* method
    
    * Merge original get_parent_image_with_package and get_parent_image, as
      they are different only from querying image with srpm_name or not.
    
    * Rename get_parent_image_* to get_image_by_layer. Original method
      tries to query LightBlue to find out image that has passed-in specific
      layer and the number of build layers, instead of getting the parent
      image of an image that has the passed-in layer.
    
    * More key information is put into ``find_parent_images_with_package``
      docstring.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>