Add common global settings for images
    
    The live_images, live_media and image_build phases have same options
    that need to be specified on each image. This leads to a lot of
    duplication in the config file. This patch adds global settings and
    phase-level settings that allow to significantly reduce duplication.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>