Rename Piwik config items after rebranding to Matomo

What does this MR do?

It renames all piwik config items to matomo, following https://matomo.org/blog/2018/01/piwik-is-now-matomo/

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on the dev.gitlab.org mirror for this branch if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• [-] Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• [-] Tests added
• [-] Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened
  • gitlab-org/charts/gitlab!1602 (merged)