Allow several env variables to be set in rabbitmq-env-conf.bat

[lukebakken]

Fixes #9258

This patch allows the following variables to be set by rabbitmq-env-conf.bat:

• LOG_BASE
• MNESIA_BASE
• CONFIG_FILE

[lukebakken]

cc @fnbk in case you'd like to test this out as well.

Test process:

• Create $env:APPDATA\RabbitMQ\rabbitmq-env-conf.bat (powershell) with this content: rabbitmq-env-conf.bat.txt

• Create C:\ProgramData\RabbitMQ and C:\ProgramData\RabbitMQ\rabbitmq.conf: rabbitmq.conf.txt

• Install Erlang from the OTP installer. This is the RabbitMQ installer I built from this branch: rabbitmq-installer.zip

• When installing RabbitMQ, use the defaults, which will install and start the service

• Note that the expected directories are created:

C:\ProgramData\RabbitMQ
> dir

    Directory: C:\ProgramData\RabbitMQ

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d----            9/4/2023 10:34 AM                custom-log-base
d----            9/4/2023 10:34 AM                custom-mnesia-base
-a---            9/4/2023 10:30 AM            179 rabbitmq.conf