Add option to disable SNI for TLS connections

[tisba]

While testing a HTTP-based system using mutual TLS authentication (= client and server TLS certificates) I ran into several issues with Erlang and errors during TLS handshakes when using TLS client certificates. The root cause is not fully understood currently, but the parts of the solution is :)

Server Name Indication (SNI) is a TLS extension allowing the server to receive a hostname indication the client wishes to connect to.

According to http://erlang.org/doc/man/ssl.html#tls-dtls-option-descriptions---client-side, {server_name_indication, disable} can be used to disable SNI:

server_name_indication, disable}

Prevents the Server Name Indication extension from being sent and disables the hostname verification check

This PR introduces the following option:

<options>
  <option name="ssl_disable_sni" value="true" />
</options>

[tisba]

In case you can take a quick look, @nniclausse, that would be great. Apart from this, I've tested this successfully and also updated the documentation.