Check if SSH is available on all the instances in a cluster.

def is_cluster_ssh_available(cluster_instances, opts):
    """
    Check if SSH is available on all the instances in a cluster.
    """
    for i in cluster_instances:
        dns_name = get_dns_name(i, opts.private_ips)
        if not is_ssh_available(host=dns_name, opts=opts):
            return False
    else:
        return True