provision/kubernetes: use separated buffers stdout and stderr attaching pods
provision/kubernetes: use separated buffers stdout and stderr attaching pods

Using the same safe.Buffer instance for stdout and stderr caused tsuru
to freeze during attach. This problem happened due to either stream
blocking indefinetely in (*safe.Buffer).ReadFrom call and preventing the
other stream from ever being read from the spdy server.

guilhermebr/tsuru,guilhermebr/tsuru,ggarnier/tsuru,ggarnier/tsuru,ggarnier/tsuru,tsuru/tsuru,guilhermebr/tsuru,ggarnier/tsuru,tsuru/tsuru,tsuru/tsuru,guilhermebr/tsuru,tsuru/tsuru