container-build: add --koji-parent-build argument
    
    Add support for a new "--koji-parent-build" argument to the
    container-build sub-command.
    
    OSBS allows users to dynamically override the Dockerfile's "FROM" image
    at build time. This allows you to build your container against a
    specific parent image without pushing changes to dist-git.
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>