luatest: copy config in cluster:build_server()

It takes box.cfg config as an argument. And changes the argument
by adding a new key 'command'. If the caller wants to pass the
same box.cfg or slightly modified to several build_server() calls,
it won't work - all options will be the same on all instances.

For example:

    local cfg = {...}
    cfg.replication = {url1}
    cluster:build_server(cfg)
    cfg.replication = {url2}
    cluster:build_server(cfg)

It will not work. Both servers will get the same 'command' and the
same 'replication'.