config: override_space is monitor's option
    
    We read override_space from [sssd] not
    [nss] section.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3068
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit fc04d11c2fdde0bfe280c6030df2b1d6bf15ce63)