Make fileserver IP binding address configurable

User may need to override download IP address that peer advertises to master. Auto detecion by `ip_addr = socket.gethostbyname(socket.gethostname())` can fail if, for example, local hostname is pointed to 127.0.0.1 in /etc/hosts on unix variants. Master could also use the address that peer uses to connect websocket, but manual override is the only sure way (think NATs and other exotic situations).

Start by locating the fileserver path that computes the advertised address with socket.gethostbyname(socket.gethostname()), then trace how that address is sent from the peer to the master. Identify the existing configuration or entry point where a manual override could fit. Done means a user can override the advertised download IP while automatic detection remains available for other users.

Written by the indexing model from the issue text.