Allow user to specify known_hosts_file

By default, host key checking is turned off. This is because if it were turned on, and the key isn't in the known_hosts file, we would either have to do interactive verification (which isn't feasible in salt-cloud) or the host would just not be accessible (and would therefore be unable to be bootstrapped).

This will remain undocumented for the moment, since turning it on will cause problems for the average user. If the provider is able to tell us the host key before we try to bootstrap it, then the provider driver can put the key in the file before calling bootstrap.