Deprecate RABBITMQ_ERLANG_COOKIE env variable support

[michaelklishin]

It's not worth the confusion it causes in practice since the server does not support it.

To test:

RABBITMQ_ERLANG_COOKIE=$(cat $HOME/.erlang.cookie) ERL_LIBS=../rabbit/plugins ./escript/rabbitmqctl status

RABBITMQ_ERLANG_COOKIE="mismatching-value" ERL_LIBS=../rabbit/plugins ./escript/rabbitmqctl status

ERL_LIBS=../rabbit/plugins ./escript/rabbitmqctl status --erlang-cookie $(cat $HOME/.erlang.cookie)

ERL_LIBS=../rabbit/plugins ./escript/rabbitmqctl status --erlang-cookie mismatching-value

Per discussion with @gerhard @dumbbell.

Closes #443.

[gerhard]

This looks good!

$ RABBITMQ_ERLANG_COOKIE=foo sbin/rabbitmqctl status
RABBITMQ_ERLANG_COOKIE env variable support is deprecated and will be REMOVED in a future version. Use the $HOME/.erlang.cookie file or the --erlang-cookie switch instead.
Error: unable to perform an operation on node 'rabbit@focker'. Please see diagnostics information and suggestions below.
...

Could we make it stand out more? I'm thinking bold and red, something like this:

$ RABBITMQ_ERLANG_COOKIE=foo sbin/rabbitmqctl status

- RABBITMQ_ERLANG_COOKIE env variable support is deprecated and will be REMOVED in a future version.
- Use the $HOME/.erlang.cookie file or the --erlang-cookie switch instead.

Error: unable to perform an operation on node 'rabbit@focker'. Please see diagnostics information and suggestions below.
...

I was also thinking that maybe we should print a warning if the value of the RABBITMQ_ERLANG_COOKIE env var differs from the erlang cookie file value. It's OK if you think that it's out of scope, but I can see how it would be helpful.

[michaelklishin]

I am reluctant to make CLI tools [explicitly] read the cookie file.

[michaelklishin]

I've made the message print

RABBITMQ_ERLANG_COOKIE env variable support is deprecated and will be REMOVED in a future version.

bold and in red, and

Use the $HOME/.erlang.cookie file or the --erlang-cookie switch instead.

in yellow to stand out more.

[michaelklishin]

Backported to v3.8.x.

[gerhard]

It stands out now on both dark & light backgrounds 👍🏻

Dark

Light

[michaelklishin]

A little hard to read the part in yellow with a white background but I guess most color choices would not be equally legible.

[gerhard]

Maybe bold instead of yellow?