Adding an IP alias to OVH dedicated server

So, as you may have guessed, recently I've moved hosting from a dedicated server at Lunarpages, to a dedicated server at OVH. Lunarpages kept the system I was on going reliably for years (indeed, I'm slowly still migrating services from it), but just couldn't match what I got included by OVH.

Anyway, to keep some things separate, I like to use separate public IPs for them. In the past, you'd just add a stanza to /etc/network/interfaces, and your work was done... but I opted for Ubuntu Server 17.10, and well - it does things differently!