Add ability to customize product name, version & banner

[dumbbell]

To override the product name (defaulting to "RabbitMQ"):

• set the $RABBITMQ_PRODUCT_NAME environment variable, or
• set the rabbit application product_name variable.

To override the product version:

• set the $RABBITMQ_PRODUCT_VERSION environment variable, or
• set the rabbit application product_version variable.

To add content to the banner (both the copy logged and the one printed to stdout), indicate the filename which contains it, à la /etc/motd using:

• the $RABBITMQ_MOTD_FILE environment variable, or
• the rabbit application motd_file variable.

The default motd file is /etc/rabbitmq/motd on Unix and %APPDATA%\RabbitMQ\motd.txt on Windows.

Here is an example of the printed banner with name, version & motd configured:

  ##  ##      WeatherMQ 1.2.3
  ##  ##
  ##########  Copyright (c) 2007-2020 Pivotal Software, Inc.
  ######  ##
  ##########  Licensed under the MPL 1.1. Website: https://rabbitmq.com

  This is an example of a RabbitMQ message of the day.

  The message is written in Paris, France.    \  /
  It is partly cloudy outside, with a       _ /"".-.
  temperature of 12°C. Wind is around         \_(   ).
  30-40 km/h, from south-west.                /(___(__)

  Doc guides: https://rabbitmq.com/documentation.html
  Support:    https://rabbitmq.com/contact.html
  Tutorials:  https://rabbitmq.com/getstarted.html
  Monitoring: https://rabbitmq.com/monitoring.html

  Logs: /tmp/rabbitmq-test-instances/rabbit/log/rabbit@cassini.log
        /tmp/rabbitmq-test-instances/rabbit/log/rabbit@cassini_upgrade.log

  Config file(s): /tmp/rabbitmq-test-instances/test.config

  Starting broker... completed with 0 plugins.

New APIS are available to query those product informations and use them in e.g. plugins such as the management API/UI:

• rabbit:product_info/0
• rabbit:product_name/0
• rabbit:product_version/0
• rabbit:motd_file/0
• rabbit:motd/0

Depends on rabbitmq/rabbitmq-common#356.

[gerhard]

Love the MOTD and appreciate the persistent term caching.

A few minor changes required before it gets my blessing.

Looks 90% there!

[gerhard]

Ready to merge from my POV, not sure if you wanted to do anything else before changing it from WIP to RTM (Ready to Merge).

[dumbbell]

I just want to move the environment variables handling in rabbit_env as said in the TODO, then I will mark it as ready.

[gerhard]