NEGCACHE: repopulate negative cache after get_domains
If SSSD starts offline the responders might only know about the
configured domain because the sub-domains have not been discovered yet.
As a result the permanent negative cache is only populated for the
configured domain.
If later the system goes online and the sub-domains are discovered or a
new sub-domain was discovered at runtime the permanent negative cache is
currently not created for those domains.
This patch repopulates the negative cache for all known domains to the
end of the get_domains request.
Related to https://pagure.io/SSSD/sssd/issue/3983
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>