Remove warning from live host check

[Zarquan]

Our deployment script starts with a sanity check to make sure we aren't deleting the live system. However, that same check always outputs a scary looking warning that suggests we are accessing the live system.

Warning: Permanently added 'live.gaia-dmp.uk' (ED25519) to the list of known hosts.

This warning is produced because we are running the live host check in a clean container and this is the first time that it has accessed the live system, so it has to add the ssh host fingerprint to the known_hosts file.

Best way to avoid this is to check for the host fingerprint and add it if it is missing before we make the ssh call.

This might not be critical, but seeing this warning at the start of every new deployment makes us inured to warnings which is dangerous. We also have to explain it to every new person who joins the team. Their first impression of the project is sloppy attitude to warnings.