Qualify ghost user attribute in case ldap_group_nesting_level is set to 0
When the sssd is set to not resolve nested groups with RFC2307bis, then
the LDAP provider takes a different path. We didn't qualify the ghost
users in this case.
Resolves:
https://fedorahosted.org/sssd/ticket/3236
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>