Fix the config options defaults for telegram: it has to start with Telegram instead of Slack

[schfkt]

/kind bug
/area config

Hi. I tried to use Telegram as output and configured it through env variables like so:

docker run --name falcosidekick -d \
                              -p 2801:2801 \
                              -e TELEGRAM_TOKEN="my-token" \
                              -e TELEGRAM_CHATID="my-chat-id" \
                              falcosecurity/falcosidekick:2.28.0-rc.0

But it didn't work. falcosidekick did receive events from falco, but it wasn't forwarded to the specified Telegram chat. I checked falcosidekick logs and noticed that the list of outputs reported at the startup was empty for some reason:

Jun 01 19:04:01 sanctuary falcosidekick[3035058]: 2023/06/01 19:04:01 [INFO]  : Falco Sidekick version: 2.28.0-rc.0
Jun 01 19:04:01 sanctuary falcosidekick[3035058]: 2023/06/01 19:04:01 [INFO]  : Enabled Outputs : []

I found the original pull request that introduced Telegram support and noticed that the defaults for the config were set with the wrong prefix Slack instead of Telegram:

https://github.com/falcosecurity/falcosidekick/pull/431/files#diff-0e426a43248661127a0c0ee115aef7a1093b635f8993b3f7ebb1dd9f05b8f249R419

I changed it locally then, rebuild falcosidekick, and Telegram output started to work:

$ TELEGRAM_TOKEN="my-token" TELEGRAM_CHATID="my-chat-id" ./falcosidekick
2023/06/01 17:59:28 [INFO]  : Falco Sidekick version: 2.28.0-rc.0-22-gef45b94
2023/06/01 17:59:28 [INFO]  : Enabled Outputs : [Telegram]
2023/06/01 17:59:28 [INFO]  : Falco Sidekick is up and listening on :2801
2023/06/01 17:59:32 [INFO]  : Telegram - Post OK (200)

I guess that setting v.SetDefault also affects proper env variables parsing? Since that's why the fix actually worked.

[poiana]

Welcome @schfkt! It looks like this is your first PR to falcosecurity/falcosidekick 🎉

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Issif, schfkt

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[poiana]

LGTM label has been added.

Git tree hash: a5ecf149591a4a44c241ad59c2d31bb0c2f946b4

[Issif]

Thanks, and sorry I missed that in my review.