responders: reset ncache after domains are discovered during startup
    
    After responders start, they add a lookup operation that discovers the
    subdomains so that qualifying users works. After this operation is
    finishes, we need to reset negcache to allow users to be added into the
    newly discovered domains.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 0528fdec17d0031996e919fcd852459e86592c35)