SYSDB: override_gid not working for subdomains
    
    The override_gid is not propagated to subdomain. This patch
    assigns subdomain's override_gid to the value comming from
    parent domain.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/4061
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>