Comment 11 for bug 1581977

Based on the comments above neutron do a proper validation of the DNS names. Nova uses the instance.hostname [1] as dns_name. And instance.hostname is set based on the instance.display_name [2]. Nova has already sanitized the hostname [3] but only considers the host limitation. So we could enhance sanitize_hostname() [3] to replace a '\.([\d]+)$' postfix with '_$group1'

[1] https://github.com/openstack/nova/blob/207d2c22538ddec4d82fafbc01e756c9d25f6e36/nova/network/neutronv2/api.py#L1497
[2] https://github.com/openstack/nova/blob/207d2c22538ddec4d82fafbc01e756c9d25f6e36/nova/compute/api.py#L1663
[3] https://github.com/openstack/nova/blob/207d2c22538ddec4d82fafbc01e756c9d25f6e36/nova/utils.py#L363