Pass server-side pluggable transport parameters to proxies

On the client-side, we pass pluggable transport parameters to the transport proxies using the SOCKS handshake (and using the username/password as a covert channel).

On the server-side, we don't have such a mechanism in place. We should spec out how this should work out. Maybe use the environment variables like this: TOR_PT_PARAMETERS="obfs2:shared-secret=asdsada;obfs3:shared-secret=paa\;ssword