Add option to specify passwordmap file

[sled]

This PR allows you to specify the RELAYHOST_PASSWORDMAP in a file so it can be used with docker secrets as follows:

docker-compose.yml:

services:
  postfix:
    image: ....
    environment:
      RELAY_AUTH: 'yes'
      RELAYHOST_PASSWORDMAP_FILE: /run/secrets/relay_passwordmap
secrets:
  relay_passwordmap:
    external: true

[gooseleggs]

Thanks