SDAP: Remove user from cache for missing user in LDAP
    
    Function sysdb_get_real_name overrode reurned code LDAP
    and thus user was not removed from cache after removing it from LDAP.
    This patch also do not try to set initgroups flag if user
    does not exist. It reduce some error message.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2681
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    (cherry picked from commit 9fc96a4a2b07b92585b02dba161ab1eb2dbdad98)
    (cherry picked from commit d0d6956c3748ba670448edd548b8edd37c5f5c1a)