The etcd peer discovery plugin crashes during startup when username and
password authentication is configured. The crash occurs with a
`function_clause` error in `rabbit_data_coercion:to_list/1` when it
receives a double-wrapped encrypted password structure like `{encrypted,
{plaintext, Binary}}`.

This change adds authentication to the etcd test suite to reproduce the
bug. The test suite now starts etcd with authentication enabled, creates
a `rabbitmq` user with password `s3kR37`, and passes these credentials
to the etcd client. The test also ensures the `credentials_obfuscation`
application starts before the etcd client to properly encrypt passwords.

The bug occurs in `deobfuscate/1` which wraps an already-encrypted
password with another `{encrypted, ...}` tuple, creating the
double-wrapped structure that causes `to_list/1` to fail.

Fixes rabbitmq#15191

The `deobfuscate/1` function wraps an already-encrypted password with
another `{encrypted, ...}` tuple, creating a double-wrapped structure
like `{encrypted, {encrypted, Binary}}`. This causes
`rabbit_data_coercion:to_list/1` to fail with a `function_clause` error
because it has no clause matching the double-wrapped structure.

This change removes the extra wrapping in `deobfuscate/1`. The password
parameter is already in the correct format `{encrypted, Binary}` or
`{plaintext, Binary}` from the `obfuscate/1` call, so it passes directly
to `credentials_obfuscation:decrypt/1` without modification.

This matches the pattern used in other modules like
`rabbit_federation_util` and `amqp_direct_connection` which call
`credentials_obfuscation:decrypt/1` directly on encrypted values.

Fixes rabbitmq#15191