InitCluster, don't use SimpleConfigWriter

InitConfig returns a SimpleConfigWriter to InitCluster, which then
passes it on to ssh.WriteKnownHostsFile, which extracts a couple of
values from it. One line later the full ConfigWriter is initialized.

By initializing it one line before we can pass the full writer to
ssh.WriteKnownHostsFile, and thus we don't need to care anymore for the
InitConfig returned SimpleConfigWriter

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>