Update grafana.ini to 5.0.0

Grafana constantly changes its default config file. Our version was
modified from the one installed by 4.1.0, which is quite old by now.
Having too many differences between our version and the default file
makes it hard to see what we changed, even if the differences are just
comments.

This commit brings it closer to what's shipped in Grafana 5.0.0.