Rename Piwik config items after rebranding to Matomo

What does this MR do?

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

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 omnibus-gitlab opened