Added note about the admin panel for influxdb_configuration.md

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

This MR just adds a minor note letting users know that the admin stanza in the InfluxDB config no longer needs to be configured for newer versions of InfluxDB and adds a link to information about the replacement admin interface Chronograf.

Moving docs to a new location?

No.

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• If working on CE, submit an MR to EE with the changes as well.
• Ping one of the technical writers for review.