Make the space override responder-agnostic
    
    https://fedorahosted.org/sssd/ticket/2397
    
    In order to make the override_space option usable by other responders,
    we need to move the override_space option to the generic responder
    structure.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit f3a5ac1a50c1fccd0801023658e42d2093e1a33a)