Cleanup /etc/hosts during node removal

It seems that commit d8470559 dropped the cleanup of /etc/hosts when a
node is removed from the cluster. I don't know for sure, but it seems
accidental. As long as we add it to /etc/hosts, we should clean it up
too.

The patch also adds two FIXMEs since this kind of modifications should
be done via RPC calls to the master's node daemon, not directly.

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