LDAP: Sanitize group dn before using in filter
    
    Each string should be sanitized(rfc4515) before using ldbsearch.
    A group dn was not sanitized in the function cleanup_groups.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2744
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit 6cb5bad3c8e2f35ca9dce1800a506d626f90c079)