Add Secret(s) Manager security risk warning messages

[roger2hk]

Towards #109, #110.

[phbnf]

This message isn't explicit about what's actually happening. I think it's important to be explicit to prevent someone from copy-pasting this config to bring up their production log. Here's an alternative suggestion:

# [WARNING]
# This module will hardcode unencrypted private keys in the Terraform state file.
# DO NOT use this for production logs.

Let's also put this "WARNING" banner in:

• the secretmanager modules main.tf files
• a README.md file in the secretmanager directories?

I really really want to make sure nobody uses this for a production log, hence the request for extra warning messages. I've created #219 to track implementation of a solution for production logs.

[roger2hk]

I have proposed to move the insecure part out of the secret manager(s) modules. See #219 (comment). After that, we can put those warning messages in that insecure module instead of secret(s) manager modules which are supposed to be safe to use.

[phbnf]

Let's add all the messages for now since that's the state of the world, and remove them once we've designed a better process to bring up new logs safely?
I'm not sure we'll get to do this before the Alpha release. If we do, that's great, and we can remove them. If we don't, I'd rather err on the safe side for now.

[roger2hk]

Ah, I've already got that done for AWS in #220 when I saw this message.

[roger2hk]

Closing this as the warning messages will be covered in #220 and #222.