daemon: Implement support for host keys when adding machine

Add a new host_key parameter to Machines.Add(). If non-empty the
key there gets added to our known_hosts file.
web: Fix 'Login with my current credentials' checkbox

This checkbox had no effect.
web: Display fingerprints in initial setup and add to known_hosts

Show real host fingerprint in initial setup and add to known_hosts
so that we can later connect to the host without problems.

We refactor the connectting to another server in PageSetupServer a
bit since we need to reconnect at various stages with different
credentials and keys.

Closes #468