Rearrange tls-related function calls and add explicit set_hostname()

[michalpasztamobica]

With #40, the example started failing on connection. It turns out that:

1. The call order was wrong - set_ca_cert() must go before open()
2. set_hostname() is required for this connection to work (our Greentea tests work fine without it).

The second point must be considered separately, as the TLSSocket::connect was removed with the recent changes in string-based APIs, but it seems it might have to stay to call the set_hostname...