Keep per-profile ssh `known_hosts`

Now that there is multi-profile support, it makes sense to keep ssh hosts in the profile-specific area, rather than mixing the hosts from all unrelated places in `~/.ssh/known_hosts`.

`~/.site/etc/known_hosts` or `~/.site/etc/ssh/known_hosts` are good choices.  `~/.site/etc/` points to `~/.site/profiles.d/ACTIVE/etc`, where `ACTIVE` is a symbolic link to the active profile.
