Don't use RABBITMQ_SERVER_ERL_ARGS

Instead, use RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS

Quoted from https://www.rabbitmq.com/configure.html#supported-environment-variables:

"
RABBITMQ_SERVER_ERL_ARGS - Standard parameters for the erl command
used when invoking the RabbitMQ Server. This should be overridden for
debugging purposes only. Overriding this variable replaces the default
value.
"

The default value of RABBITMQ_SERVER_ERL_ARGS on *nix is:

+P 1048576 +t 5000000 +stbt db +zdbbl 128000

So currently, if ipv6/tls is enabled, and the caller does not provide
its own default value of RABBITMQ_SERVER_ERL_ARGS, it will end up
overwriting those server defaults.