IPA: process non-posix nested groups
    
    Do not expect objectClass to be posixGroup but rather more general
    groupofnames.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2343
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    (cherry picked from commit bc8c93ffe881271043492c938c626a9be948000e)