Common improved slack notification

What does this MR do?

• Adds common slack-notification job
• Improve notification message
• Extract gitlab-qa installation script

Check-list

• Verify the change in all affected pipelines.

Tested via:

• https://ops.gitlab.net/gitlab-org/quality/staging-ref/-/pipelines/1159784
• https://gitlab.com/gitlab-org/quality/nightly/-/pipelines/521197152

Closes: gitlab-org/gitlab-qa#597 (closed)