launchpad-ftpmaster-publisher charm should populate known_hosts

[Colin Watson]

lp:ubuntu-archive-publishing SSHes to several other machines to trigger mirrors. We should work out a way to populate known_hosts for those machines so that sysadmins don't have to do that manually when redeploying ftpmaster-publisher. (Possibly lp:ubuntu-archive-publishing should provide some known_hosts fragments for the correct machines in some defined place, and then the charm should install those in ~launchpad/.ssh/known_hosts.)