increase default proxy timeouts for nginx<->jenkins communication

[vdelendik]

recommended /jenkins proxy settings to test and apply are:

        proxy_connect_timeout   60;
        proxy_send_timeout      300;
        proxy_read_timeout      300;
        proxy_buffers           32 64m;
        proxy_buffer_size       64k;
        proxy_next_upstream    error timeout http_502 http_503 http_504;
        client_max_body_size    64m;
        client_body_buffer_size 64m;

potentially it should improve swarm agent communication via nginx and make connections much more stable (dynamic ECS and fargate clusters only)

[vdelendik]

closing as won't fix. latest swarm agents are much more stable