dnf: apply the /etc/dnf/dnf.conf configuration file in the installer
    
    The 'gpgcheck', 'skip_if_unavailable' and 'best' are the only options
    for which the default Base() config value (which anaconda has been
    using) differs from the configuration in /etc/dnf/dnf.conf.
    
    For the 'best' the goal is to use the value specified by the config file
    to resolve the bug 1899494 caused by change of the Base() config default
    by dnf.
    
    For the 'gpgcheck' and 'skip_if_unavailable' the patch overrides
    /etc/dnf/dnf.conf to the values keeping the current behavior. The
    options are not exposed in Anaconda configuration in this patch as the
    value is the same across configurations.
    
    Resolves: rhbz#2053710