Make tailssh ask LocalBackend for the SSH hostkeys, as we'll need to
distribute them to peers.
For now only the hacky use-same-as-actual-host mode is implemented.
Updates #3802
Change-Id: I819dcb25c14e42e6692c441186c1dc744441592b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>