[Neco, Sabakan] Assign and use fixed IP address(es) on boot servers independent from rack number

[morimoto-cybozu]

What

Currently Neco assigns node0/eth0/eth1(/bastion) addresses to each boot server according to its logical rack number.
cf. https://github.com/cybozu-go/neco/blob/release-2023.09.01-24027/docs/installer.md#post-installation-setup
cf. https://github.com/cybozu-go/neco/blob/release-2023.09.01-24027/installer/setup/network.go#L135
cf. https://github.com/cybozu-go/neco/blob/release-2023.09.01-24027/address_util.go#L13
The node0 addresses of the boot servers are registered as the targets of DHCP relay on the top-of-rack network switches.

It is not desirable to manage many and various node0 addresses.
There is a functional limit to the number of the DHCP relay target addresses.
It is inconvenient if we need to update the DHCP relay configuration whenever we replace active boot servers.

We want to use a fixed set of IP addresses to configure DHCP relay.

How

Assign a new virtual IP address to the boot server.

It is not sure if we can use only one VIP or we need the same number of VIPs as the active boot servers.

Checklist

• Finish implementation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions