Support project-local known hosts file.

[leehambley]

I didn't know (until reading the docs) that SSH supports multiple known hosts files, how cool would it be if we could check in to source control (with all the integrity benefits that come with it) the list of known hosts for a given project, and refer to that list when deploying/working with sshkit?

[leehambley]

/cc @seenmyfate might be that we get support for this into Cap accidentally in the next week or so