Add ability to customize product name, version & banner

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

[#170054940]

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