Provide an alternative to -setcookie

[gerhard]

In 3.6, the Erlang cookie can be set via environment variable. For the CLI, it's RABBITMQ_CTL_ERL_ARGS, and for the broker it's RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS.

In v3.7.0 Milestone 14 -setcookie in RABBITMQ_CTL_ERL_ARGS is no longer respected. After a quick chat with @hairyhum, it turns out that this is no longer supported in this generation of CLI tools: -setcookie works for the Erlang VM but not escripts. We believe that introducing a new environment variable, such as RABBITMQ_COOKIE, would solve this issue. A regular CLI switch is also an option.

Thoughts?

[michaelklishin]

Please make it RABBITMQ_ERLANG_COOKIE. We don't want to invent a new term.