Bootnode's IP address keeps changing

[tomaka]

It turns out that #433 was a bad idea, and the bootnode's IP address keeps changing.
I'm unfortunately not super experienced at this.

[tomaka]

I've manually assigned a static IP on DigitalOcean, as a work-around for the bad configuration.

[tomaka]

After look a bit into it, I think the best strategy is:

• In the code, use /dnsaddr/ multiaddresses for the bootnode addresses. This is unfortunately not supported by rust-libp2p yet.
• Deploy a DNS server that specifically handles these /dnsaddr/ addresses. It took me a couple minutes to find the specs, but here it is.
• Update the configuration of the DNS server if a node changes IP.
• Derive all the private keys from all bootnodes from a single secret combined with the index of the bootnode. This is already the case as of Prepare a second bootnode #489

In the meanwhile I'll add bootnode: 0 and bootnode: 1 labels on the Kubernetes nodes, so that pods get assigned their proper IP address.

[tomaka]

cc #638