RabbitMQ should advertise the SASL mechanisms in the order as
configured in `rabbitmq.conf`.
Starting RabbitMQ with the following `rabbitmq.conf`:
```
auth_mechanisms.1 = PLAIN
auth_mechanisms.2 = AMQPLAIN
auth_mechanisms.3 = ANONYMOUS
```
translates prior to this commit to:
```
1> application:get_env(rabbit, auth_mechanisms).
{ok,['ANONYMOUS','AMQPLAIN','PLAIN']}
```
and after this commit to:
```
1> application:get_env(rabbit, auth_mechanisms).
{ok,['PLAIN','AMQPLAIN','ANONYMOUS']}
```
In our 4.0 docs we write:
> The server mechanisms are ordered in decreasing level of preference.
which complies with https://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-security-v1.0-os.html#type-sasl-mechanisms
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