Try to get slack deploy notifications running again. Used var name from .gitlab-ci.yml / documentation

What does this MR do?

Try to fix the slack deploy notificatoin script as I'm thinking about using it in the #foodsharing-beta channel.
As @nicksellen pointed out the pre defined variables I compared the ones which are marked as unresolved and found all of them in the gitlab ci documentation. The one I changed does now fit to the documentation.

This might be the only bug causing the script to stop? We'll see :) Can't check if the webhook url is correct since it's excluded from git and only on the server.

https://docs.gitlab.com/ee/ci/variables/predefined_variables.html

How confident are you it won't break things if deployed?

Has been broken before. Nothing to break.

Links

• Details in slack: https://yunity.slack.com/archives/C1T1ZM9A4/p1560102737160600

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))