Add support for loading FLOWER_OAUTH2_SECRET as a secret

[amirlazarovich]

This will allow one to do the following:

docker-compose.yml

version: "3.9"

services:
  celery-worker:
    ...
  
  celery-flower:
    image: mher/flower:1.2
    command: celery --broker=$REDIS_URL flower --port=5555 --auth=".*@my.domain"
    ports:
      - 5555:5555
    environment:
      FLOWER_OAUTH2_SECRET_FILE: /run/secrets/flower_oauth2_secret
    depends_on:
      - celery-worker
    secrets:
      - flower_oauth2_secret

secrets:
  flower_oauth2_secret:
    file: secrets/flower_oauth2_secret