Introduce config defaults, valid for all toddlers
    
    This introduces a default section which can be overridden for any
    toddler individually. The dictionaries will be merged at depth, i.e.
    sub-dictionaries don't have to be copied wholesale just to override a
    single value.
    
    Additionally, it consolidates the configuration keys for
    Pagure-as-dist-git to `dist_git_*`, this was inconsistent between
    different toddlers. The old `pagure_*` keys will be used as a fallback
    for the time being, if `dist_git_*` keys are missing, to support
    deployed configuration.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>