Make server_tokens configurable per virtual-host

[alekna]

Makes server_tokens configurable per virtual-host without need to build custom images or mount custom configuration.

Sample docker-compose:

version: '2'

services:
  nginx-proxy:
    image: nginxproxy/nginx-proxy
    ports:
      - "80:80"
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock:ro

  whoami:
    image: jwilder/whoami
    expose:
      - "8000"
    environment:
      - VIRTUAL_HOST=whoami.local
      - VIRTUAL_PORT=8000
      - SERVER_TOKENS=off

Fixes #1094 #1418

[alekna]

It's quite commonly requested feature and something I'm also looking to have as a built-in feature in nginx-proxy. @buchdag what are the odds of this change being accepted? Is there anything you would like me to improve in my PR?

[buchdag]

@alekna I made a review on this PR with two very minor changes, as soon as those changes are made I'll merge this.

[alekna]

I don't see any changes requested. Would you mind sharing it with me so that I could make adjustments?

[buchdag]

My bad, I did not submit it.

[buchdag]

LGTM