AD: Remember last site discovered
To discover Active Directory site for a client we must first contact any
directory controller for an LDAP ping. This is done by searching
domain-wide DNS tree which may however contain servers that are not
reachable from current site and than we face long timeouts or failure.
This patch makes sssd remember the last successfuly discovered site
and use this for DNS search to lookup a site and forest again similar
to what we do when ad_site option is set.
Reviewed-by: Jakub Hrozek <firstname.lastname@example.org>