Fix menshen agent PUT request for bridges

• remove udp as allowed transport for obfs4 bridges. Used to always put udp as transport in case the server is listening to incoming openvpn udp connections.
• menshen-agent used to send the "$hostname" of the gateway server and therefore potentially exposes details of the server. Now the hostname is taken from a new environment variable.
• instead of the obfs4 cert menshen agent has sent the obfs4 public key to menshen, which resulted in connection errors on clients. That has been fixed